Strange behavior of the IDE?

[Graham Samuel]

I'm trying to debug a sequence of code with some repeat loops and stuff in it. I'm using LC 6.5.2 on Apple Mavericks 10.9.1 - but I've just tested this on a far older version of LC (5.5.4) and I get the same problem:

I have put a number of breakpoints in the code, and when I start executing it, the breakpoints do indeed stop the code running as expected. But then I come to a point where the code seems to rush ahead, taking no notice of the breakpoints. Specifically, I start tracing before calling a certain function with this statement

put myFunction(myParameter) into field "myField"

The first line of the function has a breakpoint in it, but as soon as execution moves to the function, none of the breakpoints within it apply. It's as if I'd clicked on the 'step over' button in the IDE instead of the 'step into', but I haven't. This is entirely repeatable. What did I do wrong? I suppose it may be a bug, but I rather dread cutting it down to a simple example. Has anyone any idea what is causing this?

TIA

Graham