EDITED FOR TL;DR:
I have an above-ground fiberglass spa in the backyard of my home. I'd call it a four-person as more than that would probably overflow the tub, but it's got seating for up to seven; my best guess as to water capacity is about 350-400 gallons.
The spa's circulation system is a combination electric pump/electric heater, which is upstream of and inline with a Hayward cylinder filter and housing. We bought a brand-new filter for it when we bought the house, as a part of the general process of making the spa usable again (it had been neglected for at least the previous winter). The filter's about a foot wide by a foot and a half tall; at least half the size of the smallest comparable Hayward filters that the HIWs and Leslie Pools keep in stock, and possibly smaller.
When the filter is new, newly-cleaned, or even taken out and air-dried for a few hours, the jets run great. However, after a period of just a few days of normal circulation with the jets "off", the water flow drops to next to nothing. From week to week the situation becomes so bad that there isn't enough pressure to siphon air from the intakes into the jets when the pump's at full (and so you don't get much massaging action; not much fun).
The circulation system is very simple; skimmer and bottom drain -> gate valve -> pump/heater -> filter -> gate valve -> jets. The two gate valves basically allow you to hold the water in the tub while you work on the pump or filter. There's no backwash capability, and there's really no good way to even fully drain the tub. This, along with a host of other little things (like, I dunno, the spa no longer being level because it didn't have a proper foundation) makes me think that some things were skipped in the installation.
Currently, the workaround when the flow is too low for a dip is to take out the filter. This isn't a great solution because the filter housing is not easy to get to with bare feet, and if we let it go then obviously the water isn't being filtered and turns cloudy and green. To clean the filter, I remove it, rinse it off well, then soak it in filter cleaner, rinse well again, and reinstall it. This works for a week and then I have to repeat the whole process, which adds up to two hours to the regular spa maintenance that otherwise takes less than a half an hour (skim, brush, net, test, balance, shock, new chlorine tab, test again).
Is there a problem here? I think so. I think either the pump isn't strong enough to get good flow unless the filter is completely clean, or the filter is too small and is gunking up too quickly. There could be something else; I doubt it's bio-slime as the stuff I use for shock is also an algaecide and it does a good job. Maybe too much clarifier (the shock has that in it too)?
- Suck it up. Maybe the filter really does need to be rinsed out weekly; the FAQ from one of the comments says to rinse "periodically" and to only chemically clean the filter once a season or so (every time you need to drain/refill the tub).
- Replace the pump with a higher-pressure one for better flow no matter the filter state.
- Replace the filter housing with one that has more filter area in the cartridge so it'll go longer before gunking up completely.
- Redo the circulation lines to add backwash and/or bypass capabilities, allowing for a more automated clean-out and/or the ability to easily ignore the problem while in the tub (even clogged, the filter does a great job keeping the water clean, it just doesn't allow enough circulation when we want the jets on full).
- Look at the products I'm using and see if any are contributing to the problem (adding too much hardness, too much clarifier compound, etc etc etc)
- Drain, clean, refill tub (the water's perfect according to the last test strip; maybe a bit high on the pH)
- Rip the whole spa out and start over (may sound extreme but there are reasons)
Pics when I can get them.