Pro Tools LE 7 (Mac OS X) Looping

[Vocode]

I'm new to using Pro Tools, so sorry if this question has been asked before or the answer is very obvious:

I recorded a bassline from a keyboard in a Pro Tools session (it's roughly 35 seconds long) and am trying to loop it for the remainder of the song. I highlighted the whole bassline recording, then grouped it (via the Region menu), then looped it. However, after the first loop has played (on beat) the second loop begins and is noticeably offbeat. I don't know how to loop the region and have it stay on beat for each and every loop. It's like the loops were slightly shifted (after I highlighted, grouped, and looped) and now they aren't synching up with the original tempo (the loops following the first one come too early or too late with respect to the tempo/time of the song). Do I need to highlight/select the initial region differently so that it matches up/transitions from loop to loop better? Would Quantizing fix it?

I'd appreciate any help and will try and clarify this if it isn't clear enough already.

Thank you.

[Phil Ogden]

I suspect that the end of the loop doesn't fall exactly on the bar/beat. In which case, the start of the next 'copy' will be similarly off as it snaps to the end of the previous region.

Trim the loop end (before duplicating) or 'copy & paste' successive regions at on the start of the appropriate bars.