Saw guard misfunction on my slide compound saw?

Sir,
I have a 210mm slide compound mitre saw with redeye laser line generator number clm210lsms.
As I lower the blade ready to make a cut, the saw guard retracts but just as I am about to cut the guard snaps shut again. If I retract it by hand it will stay open. Any ideas please.
H.W.Platt.

Mr H.W.Platt
November 2009