These are (or have been) age-old problems.
You need to encourage the Theatre staff to take responsibilty for their equipment ... and get them to present it (that is, make it available) when PM falls due. In other words, put the onus on them.
Yes, it is indeed usually better from the "customer service" point of view if PM on theatre-based equipment is done (by arrangement with the Theatre staff) outside of normal hours.
Biomedical Engineering is a support service; if the in-house biomeds aren't up for "out of hours", then the Theatre Management may want to try looking elsewhere to get their PM done!
Bottom line(s):-
1) "Engage" with the Theatre Staff
2) Hire biomeds who have an interest in serving the Hospital!