Worm are losing rolling phenotype

The worms were injected with GFP reporter construct along with pRF4 plasmid (50ng/mkl). When the line was generated, 80% of the worms were GFP expressing and had rolling phenotype. A month later 80% of them still had GFP expression but almost all worms lost rolling phenotype (only 5% roll). In other words not every GFP expressing worm rolls. Does anybody know why did worms lose rolling phenotype over a time? Thank you.