I wasn't there but maybe you missed the position of the control column. The Column might have been in a pitch up position hence not allowing nose down trim. Switching on either elec hyd pump brings control column to neutral position hence allowing trim both ways.
Even a negligible pitch in any direction inhibits trim in the opposite direction. When it happened to me the first time, completely threw me off until the tech guy told me about the feature.
Try it next time. Cheers.