Hi Ben,
Had have this discussion already some ages ago. In general it's not possible to perform a L/G retraction/extension Test with only the Yellow EMP + PTU, due to restricted flow delivered by the PTU.
Available flow minimum on the green system (considering the internal leakage) = approx 9L/min
Available flow for the LDG retraction = approx9L/min at 2175psi (<3000psi)
This would mean difficulties in retraction, and possible pressure drops with priority valve closures.
But this would still not answer the question why the PTU didn't came online!
What my suggestion would be:
The G EDP pressure switch detects a low pressure, the PTU ECAM indication is displayed showing a transfer from Yellow to Green Hydraulic System. If it is a genuine low press, the PTU will start at the same time, if it is a spurious low press (Press switch failure), the PTU will not start refer to AMM 29-23-00.
So i suggest to follow up the AMM procedure again to ensure that the Aircraft would be in proper Flight mode and Hydraulic pressure is applied by a external source.
Just to heads up on the ground mode, try to apply the Parking Brake or disconnect the N/W Steering with one ENG Master ON. If the PTU turns offline i see would suggest that the flight mode was not correctly applied to the A/C.