Several things to check. I'll go by easisest to hardest:
1. Check to make sure the filter is clean.
2. Check the diverter valve is in the right posistion for the jets you are using
3. check the air valve controls, on some spas this is the collar of the jet itself
4. check the air valve hoses and check valves, spiders can lay eggs or spin webs that clog them
5. if it's none of the above, take apart the jet pump and check the impellor.
6. Call a service tech.
If you tell us the mfg. we can be more specific with answers.