vsr 10 g spec jamming back...help

[glenbrooks]

hiya lads , now and then when i have the bolt all the way back , it will not push forwards, it will lock back, either i have to wiggle it or take out the mag, and gently pull the trigger and allow under pressure it go forwards, i have looked inside and do have both white rings, i have
laylayx zero trigger, spring guide and sp150 spring, anyone know what it could be and how to fix it???? one lucky chap got away last week due to this stoppage , thanks glen