low cylinder compression

hey there,

I have a nissan silvia 88' ca18det, the engine has been built with forged pistons and a 1.6 mm head gasket.

the garage reckons I've got low compression in one of my cylinders;

1= 150
2= 150
3= 150
4= 125

all rings and bearings are new, could you please advise me on what to do, or what actions I could take,

cheers. :D