Welcome, Richard, to the OEM forum.
Pulse counting uses interrupts to the AVR-DB processor, and as the pulse is long enough for you to see, I don’t think this is the problem. It is much more likely the light level is the problem - the optical pulse sensor is purely an amplifier, and we also know that the output goes “high” before there’s enough light for the LED “repeater” to come on. This makes me wonder whether ambient light is reaching the sensor, and you have a permanent logic ‘high’ - or high enough not to register a ‘low’, if that makes sense. If your meter is in a dark place, them I am wrong.
You might find these topics useful:
Measurements & circuit diagram: First try with EmonPi - Pulsecount stuck at 1 - #16 by Robert.Wall
Also Adjusting sensitivity of Optical Pulse Sensor - #21 by Kempson