It does seem odd. If the light at the receiver is flashing then the problem can only be the connection to the flash or the opto thyristor in the receiver - which is unlikely as you have the same problem with four units.

There is no point trying it with different cameras as even the manual test button on the transmitter will check the operation.

You won't be able to measure anything with a multi-meter as the trigger voltage is provided by the flash, not the reciever.