Bseries only

My gsr hydro trans has recently started giving me problems, so when i am at a red light sometimes it decides to go into gear fine other times it does not. When it decides not to go in i try to put it into everyother gear until one go in then it will shift to 1st fine right after . It will shift fine while car is off and itwill shift 100% fine while car is moving. I added fluid and still did not work , it did help a little. Anyone know what it could be?? This is my daily driver and it does get annoying sitting at a grenn light untill it goes into gear .