This happens when the bottom of the tank has a lining of sediment [mud].
Attach a garden hose to lower tank drain. run hose outdoors on a serfuce You can see the flushed deposits drain, concrete. flush for at least 10 min.
When I install a w h I write on it with a magic marker flush each- labor day 5 min., or nearest holiday to install date. Are You on a well? I love N. H.