CustomEventHandler will only be called when there is an activity on the BLE side. Such activities include connection, disconnection, advertisement/scanning start/stop, write request/response, MTU exchange request/response, etc. Unless it is a procedure that requires application intervention, the BLE stack will take care of the activities and the CustomEventHandler will not be called.
For what activity are you expecting the CustomEventHandler to be called? Are there any interrupts in your design that have higher priority than bless interrupt and are being called frequently.?