Has anyone had a problem with a PIC refusing to take pulsed input from a particular source? Some specifics:

* PIC being used to read 1-2ms pulses from a radio receiver
* Software and hardware always work perfectly for one model of receiver and never work at all for a second model of receiver. Both receivers made by the same manufacturer. This has been tested by using the same board/PIC/power supply and changing only the receiver - many times. Both receivers work - servos will accept commands from each.
* Scope indicates a nice 1-2ms pulse at 2.93v (high), 0 (low) for one, 3.13 and 0 for the second (working one). I think frequency is about 100hz, but I'm not sure on that one.

Pic 12F675 operating on a 5 volt voltage regulator, GPIO.2 used for signal input. Common ground. wpu on the input line is off. Again, the software works well for the second and does not recognize the first at all.

The data sheet seems to indicate the voltage levels are appropriate for communication and the leakage (amperage consumption for PIC communications) looks very small. Wiring goes receiver, resistor, input pin

Am I missing something obvious? Suggestions, search terms, links and experience are welcome.