Quote Originally Posted by thomae
Look at the heads with a magnifying glass. Make sure they are not actually Phillips heads, torx heads(possibly) or square heads (doubtful). I have confused the type of fastener a few times especially with very small ones. There has to be an easy explanation.

The hard explanation is that the heads are stripped or that your wrench has the corners rounded off.
FWIW: I just checked several Weaver brand rails I have and the flat base (model 413T, Weaver number 48329) one that is NIB has slotted screws, the 20 MOA rails have torx headed screws....