Hello. New pool owner here. Our new pool has been open for close to two months with no issues. Today after my son had about 15 friends over, I noticed the skimmer weir flap was removed and left on the pool deck. I also noticed that the water coming into the pool from the return jet was sputtering and they were bubbles. I cleaned out the skimmers. Cleaned out the basket in the pool pump which had some debris in it. I back washed the filter a couple of times as well. The sputtering and bubbles is still happening. The gauge on the sand filter reads at a steady 21. Any suggestions would be appreciated. Thank you.