What a great investigation you have done!
From what you say, everything points to the fuse.
ITrip current is the minimum current required to interrupt current flow through the circuit at +23 °C. At this current level, the PTC resettable fuse is heated sufficiently to switch into a high-resistance state.
IHold current is the maximum current the PTC fuse can sustain for long periods of time (4 hours or longer) at +23 °C without tripping.
Your fuse is setting the motors power circuit to 5A maximum instead of 7A. If you used the Stepper Online motors, each one draws slightly less than 1Amp. As the robot has 7 motors, the circuit that powers the motors need about 7Amps. These are all approximations, depending on the model of motor you have used there may be slight variations in the total amperage needed.
If you don’t find a PTC resettable fuse of 7A you can use a normal fuse of 7A. It would be harder to change if it breaks, but will do the job.
Hope it helps!