When i scrub to my video i see only the 'flagged' frames. but in some of my
flv's the ammount of frames that i get to see is so low. How can i get more
'flagged' frames in my flv's?

I hope you understand what i mean, the amount of flagged frames is critical
for the app that i'm making.

The term you're looking for is "key frames". You can alter the key frame interval when you encode the flv.