1. Make sure you aren't missing parts: page 8 of the StarClear manual

2. If your cartridge is NOT Hayward OEM or Unicel, order one. (You should have 2, anyhow, so once can be cleaned properly)
Hayward CX1200RE
Unicel C-8412 Replacement Filter Cartridge for 120 sft Hayward CX1200RE
Which ever one you get, INSPECT it when you receive it, and make SURE it's an actual Hayward or Unicel and not something else being shipped as 'equivalent'.

3. Check your piping and ID ANYTHING that bypasses the filter (like an off line chlorinator, etc.)

Get back to me of those 3 steps don't ID the problem. I've used StarClear filters to build cart-based vac systems . . . so I'm pretty sure your problem is not typical.