You should not have to buy a whole new unit if you can do some basic troubleshooting to narrow down the problem. Every part in those things is replaceable, and none of it is magic, so it's just time and effort.
If everything is moving around the pool correctly, then the problem is most likely with the little venturi jets inside the middle part of the unit, which fire water toward the bag. Any small debris like sand or dirt or some chemical that makes it past the filter can do this. It's also not very good with air in the returns, but I doubt that's causing this issue. With the unit connected, hold it above water and see if those venturi jets are firing. If not, or if they aren't firing with some force, then you can disassemble the unit and try to clean those jets out manually (usually canned compressed air works, or maybe a thin object to poke through the opening (but be careful). Also check all the internal hose connections. And remember if nothing else, all those parts you see are replaceable - the parts breakout is in the manual and pretty much everything is well priced on Amazon, or available at your local pool store.
It could also be that the jets are working but the water leaks out before really hitting the bag - not really sure exactly what would cause that but something to look for.
To prevent these sort of issues in the future, make sure the little filter screen on the return fitting is present and clean at all times. This should catch stuff so it doesn't go further into the unit. And make sure you disconnect the hose prior to any backwashing or other filter operations like turning the pump on and off. Speaking of, make sure your return pressure is still good - those are the most "difficult" jets to fire, so pressure matters a lot in this case. Check the filter pressure, valves pointing to returns, the pressure relief valve on the Polaris return fitting, swivel joints aren't leaking, the backup valve works, etc. Eventually something will stand out as the likely culprit.
