/function doesn't work

I can't type /function a:b in game where a is a folder in functions folder and b is b.mcfunction file in a folder, game outputs "Unknown function 'minecraft:b'".
Additional info:
-files aren't empty, they're filled with commands tested on commandblocks
-files are written without / are propertly loaded by minecraft as seen in game output
-game sees those functions, as they're accesable via tab-autofilling