I've spent $100 in tournament buy-ins but my welcome bonus has not updated - why?

Each tournament entry is made up of two/three elements, the buy-in and the rake/fee. The buy-in is the part that goes towards the prize pool and the rake/fee goes to the poker room. 

For bonus clearing, it is the rake/fee part which counts towards bonus clearing. 

The third element occurs in a knockout tournament, the buy-in is made up of three components, the buy-in, the bounty prize and the rake/fees.

Here are two examples to help explain things better:
  Sunday Special $22
  $31.50 Bounty Hunter

As you can see the buy-in for the Sunday Special of $22 is made up of $20.24+$1.76 - the $20.24 goes to the prize pool and the $1.76 is the rake.

In the $31.50 Bounty, there is $15+$15+$1.50, the first $15 goes to the prize pool, the next $15 goes towards the bounty and the $1.50 is the rake.

If you play the above tournaments you will have raked $1.76+$1.50 = $3.26 which counts toward your bonus clearance.

For Spin & Golds, the rake is 7% so if the buy-in is $3 it is made up of $2.79+$0.21 with the $0.21 being applicable for the bonus clearing.