Sometimes I have this thing happen to me, and I just can't understand what's going on and why.

I have a simulation. I don't have prior checkpoints saved. I have checked the folders. I tried changing directories and filenames, enabling/disabling cache, you name it.

My simulation plays back. It's just POP particles. I click play, and it sims. If I have memory or normal disk cache, the blue bar on the timeline fills up and I can scrub it.

However, as soon as I check “save checkpoints”, the bar goes orange, and if I play it, it just breezes through the timeline, no particles spawning. I tried it with various paths and filenames, I tried leaving it blank, nothing helps. If I remove the tickmark and reset the simulation, it plays back and sims again - until I try checkpoints one more time.
At most it writes the first - empty - frame, and nothing after that.

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Another problem:

This happens with rigid bodies, FLIPs, and mostly Pyro. I have a sequence of .sim files. Sometimes out of the ROP render, sometimes out of the checkpoint cache with 1-frame increments. I plug it into the DOP network's Simulation PLayback knob. And if I'm lucky, it plays. Often, it just doesn't play at all. With my particular Pyro sim, it plays only the first 3 frames, then just keeps the 3rd frame for the rest of the timeline. If I plug in the frames individually instead of the sequence (file_0014.sim instead of file_$F.sim), each of them displays correctly - so the sim isn't corrupt or anything.
The only fix I found so far is to use the Checkpoint option for the same file sequence instead of the Simulation Playback, but it's slower and even then also buggy.

Anyone have this issue? It becomes a big pain in the ass if it's a large simulation that takes a long time to calculate.

Your second problem would require a scene file to diagnose. Regarding your first issue, I assume it's just some incorrect setting in the .sim file cache settings. I'd need to see your exact settings to see what might be wrong, but off the top of my head, perhaps you haven't increased the checkpoint trail length to the length of your simulation? If you have this at the default of 0, then you will probably get the behavior you are describing.

In regards to the first issue - I indeed have it set to 0. I thought that was supposed to make it “as long as needed” - should I specify a specific number? Also, I've found out that if I erase the cache URI completely, press enter, and then type in a new path, it increases its chances of working…

Current scene is pretty big and messy, but if I manage to isolate the issue I will save it and upload, definitely. It's just that it's so temperamental - with the same sequence, sometimes it won't play no matter what - fresh file, reboot, you name it, then in a day or so it will work just fine.

I feel like there's just something I don't understand about Houdini's caching maybe.

You need to make the checkpoint trail length the entire length of your sim, that's the only way the cache will work properly. Also, one thing to note is that if you interrupt your sim, and want to resume it, if the last frame you have is incomplete, it will break the sim. What you want to do is delete that incomplete sim file so that the last .sim file is full size, then ctrl+click the brain icon - the playbar should turn blue for a properly cached sim, and you should be able to resume from that point.