Intex Sand Pump Low Flow (Solution in Video Description)

EDIT: SOLUTION FOUND. The problem was the algae clogged up the filter. So after doing the SLAM process and backwashing every couple of hours, everything is back to normal.
Make sure you KEEP THE WATER AT SHOCK LEVEL with enough bleach or tablets. I want to thank the folks at www.troublefreepool.com for their help.
Problem: The pump runs perfectly fine on recirculate mode, which bypasses the sand. The PSI is in the green, between 5-7 PSI.

However, when I run the pump on filter mode, the PSI is about 11-12 PSI, and after 30-60 minutes, the water return to the pool is much, much weaker.

Things I have done, so far:

- Backwashed, and rinsed the pump several times, until the water runs clear.
- Removed the top of the filter and washed it.
- Wash the sand with a hose (while covering the center tube).
- Clear the basket of debris (there was none).

Next, I'll replace the sand, but I don't think that's going to help.

What I don't understand is why it runs fine in recirculate mode, but weak on filter mode? My guess is the inner tubes under the sand are clogged.

I'll update this video if I find a solution.

Видео Intex Sand Pump Low Flow (Solution in Video Description) канала six25