step through

Today, for some reason, the "step through" function of the debugger has 
stopped working in my LWL (it simply steps across the function, without 
entering and stepping the called function - i.e. acts like "step").

Since it was working fine a day or two ago, I've obviously done something to 
cause this to happen - but I can't figure out what.

Has anybody seen this before?  Got any hints?

thanx
pt