Well, you must have changed something. Perhaps enabled OpenGL? Applied a layer style? Added an adjustment layer somewhere? Something is breaking your render order, that's a fact. Double-check those things.
thanks for reply very helpful.
you're right I did change something, I applied the glow effect to 1 of the nested comps and that's when the transfer mode stopped working (or maybe that's just when i noticed it had stopped working). not just for that nested comp but for all of them
turning the glow effect on and off again or deleting it made no difference. i tried turning openGL on and off, no difference.
when you said something had broken the render order I thought why not just import the whole lot into a new after effects file and bingo everything is fine again.
oh joy I think this job will be delivered on time
Problems are often the result of the LAST thing you did. Look there first. I'd cut the Glow effect from the pre-comp that's causing the problem and put it in a comp of it's own.
Rick has a point and the otehr alternative is to add yet anotehr level of pre-composing without collapsed transformations and apply the effects there.