stubs

Hi all, I'm new to hot tub ownership, so please bear with me. I'm living in the UK, and have recently been sold a hot tub by a family member; It's a Watkins Hot Spot RLX, and I believe is approx 12 years old (maybe older?). Was planning to buy one new in a couple of years, but we couldn't say no for the price we paid. It will certainly do for the time being. We transported the hot tub to my home, and located it in the back garden. Gave it a good clean up, and filled it ready for use. It warmed up nicely, and the pump is a little bit on the noisy side, but nothing we can't live with for now (I believe it's been refurbished once during the tub's lifetime). The tub has 4x directional "hydromassage" jets (all missing the screw-in nozzles, which I have on order), and 6x "precision" jets. I've noticed that the 4x large jets flow water fine, but don't create any bubbles - are they supposed to? Also, out of the 6x precision jets, only 2 seem to flow any water, some of them trickle a bit, but mostly feel completely blocked. The 2 that do shoot water, only create bubbles now and again, and I find that to get them started, you have to cover 3 or 4 of the others up. Even when they are producing bubbles, one only does half the amount as the other. Adjusting the "Jet Comfort Control Lever" does turn them on & off, but they don't work as expected. I've had a read up on a few sites & searched for a solution, and thought I'd have a try with some "Swirl Away"... I gave the hot tub just over an hour running, and actually used a full bottle (half at first, and the rest after 30 mins), but it hasn't made any difference. A little bit of stuff came out, and the water was very smelly when we emptied it, but it didn't go brown or foamy like I've seen on photos and videos. The hot tub was re-filled as per the instructions, and there doesn't seem to be any air lock. The filter is nice & clean, and the jets still don't bubble correctly. Is there anything else I can try myself before calling out an engineer to come look at it? Is it worth trying to clear out the pipes using some pipe cleaner rods? Or is it more likely that the jets are actually in need or replacing? I did also try taking off the check valve to see if that made a difference, and it didn't. Please help! Thanks, Stu