I would agree with Jeremy ...

I would agree with Jeremy Biddle that these are likely to be the causes. I would also add from experience it is worth checking the following:

1 the isolation valve is sealing during the backwash cycle. It is a relatively cheap fix to check and clean.

2 Then check the backwash water rise rate with the air is sufficient to fully expand the bed for the average sand grain installed.

3 Finally check there is no sand in the under floor obstructing the water and air flow.