Have a problem that I'm not sure of the answer to.

I have a workshop/garage that has almost entirely cinder block walls. When we bought the house, I knew I'd be doing a lot of drilling into the blocks, so I bought a very good rotary Hilti Hammer Drill.

The drill goes through the blocks with ease. Much better than my previous Makita (which was no slouch either).

The problem is, that many of the holes are slightly too large, and often, Tapcons are too loose in the holes. I've tried, at times, to use one size smaller drill bit, but it seems almost random when the holes are too big. I'm not drilling massive numbers of holes, just occasional use.

Is it a dull bit (It has happened with a few different sized holes, and I have replaced the bits)? Some other technique thing? Something I'm missing?