The main problem is that capitalism is the dominant economic system, so even when it fails and states collapse and billions die for no reason other than "line go down", the successor system is usually still capitalism (or would be perceived as such by your average person). That later point is important as well: people think capitalism is just when you buy and sell things, like when there's a market. The person you're talking to might have no idea what defines capitalism in the first place, so you might be having two different discussions in your heads.

The important thing is to have clear ideas of what constitutes failure. Multiple categories in your own head with examples locked and loaded. Folks here have already given good examples. I'll try adding a few.

• Allegedly stable system shits itself every 8-12 years and has to be rescued by the state at the cost of the common person. AKA, "the business cycle", predicted by Marx among others over 150 years ago.

• Is in a constant state of failure for the vast majority of humanity, as despite producing so much stuff and food and transportation, it fails to distribute equitably. In fact, by design it distributes inequitably and creates a regime in which countries, regions, continents are starved to maintain economic hegemony. Example: the US forcing poor countries to deindustrialized and import food. You can even see this in the imperial core, where children still go hungry.

• Imperialism generally. Those poor countries forced to take on food dependence are nearly universally poor because they're forced to undersell their labor and adopt a colonial extraction regime where foreign interests own their production. Countries that try attempt to reject this system are characterized as dangerous, socialist, and targeted for destruction. During the Cold War, this was often a self-fulfilling prophecy, where a country would forcibly buy back its own assets (land, natural resources, factories) that had been stolen under colonialism, then the US would sanction them and train + arm oppositional paramilitaries to coup that country, forcing them to work with and buy weapons from the only other superpower, and one willing to deal with them: the Soviet Union. Every country that tries to go food independent, does basic land reform, tries to turn its natural resource extraction into a domestic driver of economic independence and diversification is targeted for death by capitalist interests.

• The rate of profit tends to fall, creating longer-term cycles of crises that result in major - and violent - shifts. The "solutions" to these crises create all-new ones. The petrodollar and its consequences. Financialization (good luck buying a house or getting healthcare). Neoliberalism generally. And, fundamentally, war, war profiteering, and "rebuilding".

• Speaking of profitability and neoliberalism: the stripping bare of social services, product inventories, life-sustaining materials. Look at post-Soviet states generally. Their services and guarantees were stripped, undemocratically, virtually overnight. The rapid reintroduction of raw capitalism was overtly monstrous and killed more people in the region than WWII, but it was social murder so the West doesn't care.

• Capitalism is failing in front of your eyes re: climate change. It is intrinsically incapable of putting people before profits, and emissions are a way to be profitable so capitalism says you can get fucked when it comes to global warming.

• Capitalism fails so hard and so consistently that it also requires a massive propaganda machine running 24/7 to convince you that everything is fine, actually. That climate change stuff? Don't worry, we'll make the cars electric and install some solar panels and it'll all be good and green. Recycle that water bottle to save the planet. Buy a Tesla and we'll be fine. You think there's a recession right now because everything's expensive and you can't find a job that pays well enough? Stupid fucking poors, that's not how Sir Paddington Goldbottom defined a recession in 1879 and by his metric of stock prices and the power to hire, we're doing great! Not getting paid enough? Get a better job, don't unionize! Can't find a better job? You're a low-skilled worker that needs to make themselves more valuable. Get in that grindset! Can't afford to go to school? Should've thought about that earlier, it's your own fault you didn't go into [current fad that requires economic stability to even begin in]. Inflation is up so the science says we've gotta get wages down, sorry those are just the facts. Those [x race] people in that country over there? They're why you lost your job, they're why your grandma died without healthcare, and they're going to get us if we don't get them first. Protect freedom and democracy! In other words: every failure of capitalism has created absurd and massive propaganda efforts to distract from the systemic issues it causes by focusing on a scapegoat: the poor, individual choices, a race, a gender or gender identity, a sexuality, a country of origin, other countries in general, moral decay generally, or my favorite: this is just how things have to be, scientifically, so that's why your community has to suffer in poverty and violence while military contractors get paid infinite money to kill brown children in school buses in [foreign country].