Frame Selection in TBS4

I have Toon Boom Studio 4 and have had this issue for a while:
I cannot (at times) select a frame or frames in an element in the drawing window. TBS4 does not acknowledge the cel or frame I am selecting and will jump to another frame or cel. Sometimes a number of frames do not select [5 in sequence or every second frame or indiscriminate]. Going to the Screen Planning window and methodically selecting each frame in the XSheet panel will temporarily allow the frames to be selected (at times) but TBS4 will not allow frames to be selected here also. Because this does not happen all the time and it will affect different frames each time I access TBS4 its frustrating to work with.