How to snap a cross disolve

Hi I was wondering if someone could help me? 

When dragging the length of a cross disolve, I was able to snap it to where the play head was, or to the start of a clip or another cross disolve, but now that no l onger works, it jjust moves freely when dragging its legnth. I think I pressed a shortcut to disable this but I can;t for the life of me figure out what I have done. 

If someone couldl let me know how to turn this back on it would be greatly appreciated, I used it alot.

Thanks, Nick