I had this problem a ton initially.
Every time I logged in, wandered too far, or swapped ages/nether/end/etc.
After I chunkloaded it, it stopped happening outside of when I log in. Seems like(guesstimate) they aren't always/properly checking if they're supposed to be running or not until the next time they would have used a bucket of biomass, so if they get interrupted they just stay off.
Sometimes flipping their redstone off and then on again 10-15 seconds later will work, most of the time not. I think that's an entirely separate visual only bug where they appear to not be running when they are on though. You can check the top right tab to see if you're outputting any energy or not, I've caught a few stationary engines still outputting 5 MJ/t.