MPEG2 encoding in compressor bitrate issues

Hi

I'm having a problem encoding MPEG2 in compressor.
I'm new to this and have read up as much as i can but still having some problems.
My MPEG2 specs are Program Stream, 640X480 square pixel aspect ratio, progressive, minimum bitrate of 15mbps, long GOP.

My work flow is..
Ripping DVDs with MPEG Streamclip to quicktime file using DV codec and deinterlacing.
Importing to FCP, keeping the sequence settings the same as my clip settings, apart from changing the field dominance to 'none' and the compression settings to 'progressive'
Then blowing up the image slightly so that there are no inactive pixels around the edges.
Exporting to Compressor customising the settings to my specs.

For some of the files I've encoded, this process has worked perfectly but for a few I can't seem to raise the bitrate higher than 13 or 14mbps no matter how high i raise the bitrate settings in compressor.

I have tried rendering and exporting to quicktime, then bringing the quicktime ref into compressor but it hasn't helped.

Can someone tell why this is?

I assume I'm using the correct workflow as it's worked for some of the videos?

Would appreciate any ideas.
Thanks
Abi