Head loss is equivalent to the friction loss (i.e. water in the pipes loses energy) in the plumbing system. The higher the head loss, the lower the flow rate (GPM) of the pump. However, as I said in a earlier post, you should have enough flow for the jets. 100 GPM should be more than enough for 4 jets.

One thing you could do is find out from your pool builder what jets he put in the spa and if there are any alternatives that will create the action you want. BTW, what has your pool builder said about this?

To increase flow beyond 100 GPM would require replacing some of your pipe which is not an attractive solution. I would focus on the jets and see if you can find something there that will be satisfactory.

One other thing to try is to remove all of the nozzles on the jets and run the spa to see if there are some obstructions that come out. There are usually two components to each jet. The eyeball or spinner near the wall of the spa and the nozzle which is deep inside the spa. You can remove the nozzle with a ratchet extension. If you don't fell comfortable doing this, I would suggest getting your pool builder to do it for you.