I'm pretty new to after effects and so not sure what exactly I've done wrong, but my composition appears different to the exported video file and even the preview in premiere when dynamic linked, it's only a few tiny changes but I've checked many times and the roto doesn't touch those areas, there is nothing that should affect those little pieces at all. Basically what I did was track some text onto the comp then use the rotobrush to add the layer on top of the legs walking past, so the text stays behind them, it works perfectly in my comp but even by exporting one single frame as a jpeg it isn't working!
Examples below of one of the few issues, notice the pieces randomly missing in the "OD".
my comp window
how it looks everywhere else
Thanks in advance!
Without seeing your comp or looking specifically at the alpha channel of the roto brush layer it's hard to say exactly what's going on but I suspect user error. The first thing that may have gone wrong would be failing to freeze the rotobrush layer. When I use rotobrush it is the only effect in the comp, then I render the matte using the output module and select render and replace as the option. If freezing rotobrush does not fix the problem or if purging everything in AE doesn't reveal the problem try isolating one layer at a time and making sure that everything is as it should be.
If that fails, then we need to see a complete screenshot of your comp showing the modified properties (press the U key twice) of the layers giving you problems. Depending on how you laid our your comp we may also need to see a flow chart or any nested comps. Screenshots of the error without the details are almost useless.