Here's the latest!

I have upped the PSU voltage and checked all the connections.With a pull up resistor the optical encoder output switches between 1.3 and 4.4 volts.

It will trigger the interrupt the first time it goes high. No further interrupts occur.
If I reset with encoder output low, then the first low low to high transition triggers the interrupt.
If I reset with encoder output high, then the interrupt is not triggered.

If I replace the encoder output with a switch to 5volts, I can trigger multiple interrupts so the software looks like it's working.

Any ideas anyone?
Thanks
Danny