Sounds like too much pressure load or looking at that trigger unit something stuck with gunk run a bowl of hot soapy water and scrub it up mate then dry throughly and spray with brunox spray and wipe all excess off should sort that
Was it cocking before?
What's if feel like when you try? As if it won't go far enough down to engage on sear?
If wasn't cocking before try take any preload out if non in you may need to shorten spring
Also ensure the piston sear is facing right way round sounds daft but seen that done before lol