It could be the "drag" of your quilt ..... the larger mass of quilt compared to what you have done before is hanging off the extension table and creating weight, and preventing you from getting the slide you want/need and are used to when doing your other FMQ work.
To check if that is the issue, my suggestion .... make a practice sandwich and see if you can FMQ without the problems you are experiencing on your current and bigger project.
Good Luck!