I recently contacted both Full Tilt and my Rakeback company about a missing rakeback payment. Full Tilt did their normal song and dance where it seems they never read your email and just send a form letter on your header.. But my Rakeback company did reply and they explained something that I was not aware of and something Full Tilt doesn't want you to know..

The minus amounts are a result of deductions made by the poker room. These
amounts carry over until they are off-set by positive rake.
This is why you have not received any rakeback for this period.

Full Tilt deduct for a number of things from a player's rake before they
calculate rakeback.

1) Bonus
2) Deposit transaction fees (you should not withdraw and re-deposit
unnecessarily)
3) Some withdrawal fees ($11.75 for Cheques, $25-$28 for Cash PickUp)
4) Items bought in the FT store (at $0.005 per point from your rake - or
$0.00135 from your rakeback 27%)
5) Any tournaments you enter where there is a guaranteed prize and there
is a shortfall in players.

I lost $120 bucks in Rake back for #5 due to Full Tilt suffering short falls in guaranteed money.. UN****ING REAL.......