The reason for the difficulties with the A319 on the Canaries and Cyprus routes is most probably the certificated MTOW. Like the original BA aircraft these ex BMI A319s are only certificated to 64 tonnes except just two aircraft which are 68T. (Data is from G-INFO) It is only these latter aircraft that would stand any chance of operating full load on these routes and even then adverse winds could be enough to cause a fuel stop.
A319s can be certificated to much higher MTOWs such as 75T on the Germania aircraft which would have no problem operating longer routes. The reason for restricting MTOW is usually to reduce landing and en route charges. Manufacturers brochure figures are best ignored in assessing the capability of an individual operator’s aircraft since customer specifications can vary widely. I am surprised BA had not identified the problem before assigning these aircraft to those routes.