Wood filler and varnish

[FirstTimeMumApril19]

I know i have gone wrong but I don't know why or how to fix it.

The builders have built a new staircase and filled in the screw areas with wood filler and sanded it down. This weekend I've varnished the staircase and have huge white circles/lighter areas where the wood filler is.

Why has this happened? How do I repair it and stop it happening next time?

TIA