Hi,
I must have been messing around the settings and deactivated the feature that lets you snap to the beginning and end of a selection, which is really useful when repeating/trimming/moving etc. so if anyone knows how to reactivate this it would be greatly appreciated.
Ensure that “Snap To” is set to “Off” in the selection toolbar http://manual.audacityteam.org/o/man/selection_toolbar.html#snap
(if the cursor is snapping to time units, then it will probably not match up exactly with clip boundaries, so you will not get the yellow “snap” lines)