WE M&P hammer problems

[Aengus]

So recently my little bird has started to not cock when you rack the slide but if you push the slide down and rack it it does. So when I shoot with a magazine it shoots a three round burst then the trigger goes dead. It has only started doing this recently which would lead me to believe something has worn and looking at my TM the silver bit which engages the slide seems lower on the we. Would this just be a case of replacing a spring or is there something that maybe broken?