Either it's '''choked with dust/grease/debris''' or it may be '''[http://www.searspartsdirect.com/Craftsman-Saw-Parts/Model-31510721/0247/0744440/00030372/00001.html|broken inside]''' (lock button or actual micro switch).
You'll have to ''examine it closely -'' use some spray cleaner, if that doesn't work take it apart and check the trigger for damage.
''If this answer is acceptable '''please remember to return and mark it'''.''