Cannot set starting time for a layer I Sometimes work sometimes not

Hi there,

I've been trying to make watercolour effect and in one step, I need to duplicate the layer, reduce the opacity and drag the starting time backward a little bit for the layer on the top, to make sure it has the 'bloom' effect, just as below.

It worked twice, but the thrid time it just didn't work. When I played it, the whole thing just lit up on half-sec key frame because of the increase of opacity. I'm not sure if the software not responding or if I did something wrong. Anyone have idea what the problem might be?

The effect I got:

Many thanks,

Moira