Issues importing iStock comps into After Effects
After Effects works better with codecs that compress each frame individually (intra-frame codecs).  FLV files compress across multiple frames (inter-frame), as do mp4 files.  If you can get your FLV successfully converted to a codec like ProRes, it should work correctly in After Effects.  FLV files are quite unreliable in conversion- there's a post here that might help - it seems most people use the (now deprecated) open source Perian codecs to get this to work:

https://forums.creativecow.net/thread/8/1038982

Your best bet is probably to find source stock material in a format that is easier to work with (most videos on iStock are now mov/mp4 which should be easier to convert)

(NB - I'm not sure this is really an After Effects question.)