Hi, I'm Mike and I have 20+ years of experience in the plumbing field. I'll be glad to assist you today. A circulation pump is used for the heating and that would likely be what is at issue here. Can you tell if this pump is running? If you cannot tell you may be able to place your hand on it and feel it running.
Have you recently filled the tub? Have you checked the filter? If you remove the filter as a test does it function then?
You have recently filled the tub though?
I am sorry if I was unclear. Had the tub been emptied and refilled before the issue started?
Is the water filled above all of the jets? It may have pulled some air in before the emptying or it could have been a filter problem before and now it may be air caught in the pump. If you loosen each union one at a time until air escapes and tighten again when it is only water and do this at the union for the filter pump and the heater. Do not over loosen, just enough that the air can escape.
It might be black.
I love it when a plan comes together!