So the title sums it all up pretty well. I'm making mods specifically for 1.12.2, if that helps.
Any time I export a mod into a .jar form, it doesn't work when I put it into the mods folder. Let me expand on that: I fire up the game in forge, with the mod in the mods folder and.... Nothing.
Now, this isn't something that happens every time. With a previous version of the mod that I no longer have, I got the game to fire up, but it wouldn't even make it to the Mojang logo. All I was presented with would be a black screen where nothing happens. If I click the window, it wouldn't respond and eventually crash, leaving no crash report.
Now with the modern version I've made and can present, I can put it in the mods folder, but when I fire up the game, it acts like it's going to start, but no window opens, though the process does run in the background.