hi I have a conventional sand filter for my 48,000 litre free form pool. I have recently (beginning of our season) cleaned up the pool which required flocking with a "Polyaluminium chloride" product. Once it had settled overnight I vacuumed direct to waste. did this two days in a row then on third day decided to clean up the few spots very small patches of flocked debris using the filter back to the pool in the normal way. Then backwashed all ok. On resetting after backwash and restarting filter to pool the backpressure went straight up to 1.1bar and I had no flow. Checked out recirculate option, backwash option both worked fine. Have since had pump and pipework apart and top off filter no obvious blocks. Filter takes two bags course and one of fine sand and water flow is to bottom gallery then rising up through bed to top discharge back to pool. What has caused this ??? Only thing I can suggest is that fine debris remains in filter sand bed in spite of normal cloudy backwash sighted and when I try forward flow to pool it binds up in some way. Water is absolutely crystal clear now. Any help appreciated.