The most likely issue is trigger delay. On the Canon A-1, 1/60s is the maximum X-sync speed, but that assumes a compatible on-camera flash system. With off-camera radio triggers, there can be enough communication delay that the flash fires too late at 1/60s.

Try using a slower shutter speed than 1/60s and test from there. That gives more margin for the trigger/receiver delay.

Also check for radio-trigger reliability issues: some triggers can have weak/dead zones depending on antenna orientation, receiver position, and distance. A marginal radio link can make timing inconsistent.

So in short:

• 1/60s may be too fast for this trigger/camera combination
• Use a slower shutter speed
• Check trigger orientation, distance, and signal reliability

Since the setup works on your digital camera, the flash itself is probably not the problem.