I found a solution to this old problem.
Credit to goneredcar1 for his detailed explanation & battery replacement video on youtube
The charging board appears to be the culprit in many instances of charging failures
The cells were at 4.1v but wouldn't charge. I separated the cells by cutting carefully through the double sided tape between them, removed all the surface mounted components, then soldered the wires on to the top of the board. Controller now charges correctly
Credit to goneredcar1 for his detailed explanation & battery replacement video on youtube
The charging board appears to be the culprit in many instances of charging failures
The cells were at 4.1v but wouldn't charge. I separated the cells by cutting carefully through the double sided tape between them, removed all the surface mounted components, then soldered the wires on to the top of the board. Controller now charges correctly