"Isn't there always a rise in pressure when the spa is on?"
It might go up. If that's what it has always done, then that's OK. If that's something new, then it could indicate a problem. What is the pressure reading when the system is on Hot Tub? Is this a two speed pump?
You have a 2 HP pump; that should deliver about 85 to 145 gpm (avg. 115), depending on the pressure. You have 8 jets; that makes it about 10.6 to 18.1 (avg. 14.4) gpm per jet, which is right for some jets. Since this is a new problem on a system that was working fine, we will assume that the design is correct.
When the system is on the Hot Tub, is there any suction at the pool skimmers; or is there any pressure at the pool returns?
Are the automatic valves working properly? As they get old, they need to be serviced (lubricated) periodically. They may be getting stuck and only opening part way, which could restrict your suction or return to jets.
You could also remove the directional eyeballs in the jets to look for missing or worn parts. Sometimes the gaskets get worn out. There will be screws or a threaded retainer ring holding in the eyeballs. Remove all of the eyeballs, run the system and note any debris coming out of the jets, also check the pressure with the eyeballs out.
Also, there is a check valve on the outlet of the blower that may be sticking. If it is not working properly it could limit the air intake of the jets, which would reduce the pressure feeling of the jets.