Low DNS Cache hit ratio

I have enabled transparent DNS cache on a F5 that is doing a DNS LTM (No GTM) and anycast via BGP injection role which was originally configure with the DNS iapp. However the cache hit ratio is much lower than expected and increasing Message Cache Size and Resource Record Cache Size have made no difference. When looking at the cache (show ltm dns cache records rrset cache )it always shows only 1000 records regardless of cache sizes.

Total records returned (tmm1): 1000

This is only a problem on higher utilization devices, those with lower utilization have much higher cache hit ratio but also always show 1000 cached entries.

Any ideas?