Hi,
anyone think of a reason this should happen, its a packed app I'm debugging and last time I looked at it about 10 days ago breakpoints worked fine so detecting olly seems unlikely to be the problem.

delete .udd file

thanks, someone had already suggested that to me but it made no difference.

move Ollydbg to other location.
if ploblem still happend delete ollydbg.ini and re setting again.
Just one idea.

Packer like asprotect clear debug register inside exception handler. So hardware breakpoint will not work after exception.

If another breakpoint is caught by ollydbg (eg, a soft int3 on system api ),
Ollydbg will restore the debug register so hardware breakpoint will work again until next exception.