Do a fuction check. Make sure it is unloaded. Hold the trigger and pull the charging handle, after you let the charging handle back down and release the trigger you should hear an audible "thunk" noise as the disconnector lets the hammer go. Military manuals states it should be able to do this 5 times in a row.
FWIW when I first got my Bushmaster A2-20 it was doing this and I ended swapping every part of the FCG until I narrowed it down to the hammer, the surfaces where the hammer catches the trigger were very rough and uneven, replaced hammer and the problem went away. Could still be the disconnector as others have said.
ETA: I am assuming you have tried putting the stock springs back in........