Groove expects a beginning and ending time, so if you want want to have a fixed length loop, you need to update both values (not just the ending (or start) time), if that makes sense.
If you share your patch I can help in more detail.
As @dan_derks mentioned, however, karma~ is built exactly for this kind of thing, and the paradigm of “position” and “window” (vs “start time” and “end time” in groove~) makes this stuff much easier, as you can literally keep a fixed “window” size, and just vary the position.
You can download karma~ via the package manager in Max7 (or here).