[Bob Brown]

There are a few things you might want to check...

Pro Tools will only loop if the play selection is greater than or equal to 1/2 second. I would guess this is not your problem, but I thought I would mention it.

Do you have your edit and play selections linked?
We loop on play selections by default rather than edit selections.

Are you in "Dynamic Transport" mode?
Right click on the play button or loop under the Operations menu. This can make looping work a little different. It takes into account the little blue triangle and where that lines up to the play selection.