I have a Polyvinyltoluene Scintillation plastic. I wonder if that works with the SiPM.
It should do both work with blueish light.
However how is your scintillator made. The one in the Tiger instrument has total internal reflection on all surfaces but one. It could be that your plastic is not coupled to the detector.
http://physics.wustl.edu/supertiger/tiger/science/instrument/scintillator/
I have optical coupling grease
That should help.