i washable glue stick works well to hold pieces- and washes out- i don't know if basting spray would hold them or not- if you have some on hand you could give it a try-
i pick up glue sticks at the dollar store in 4-packs- very inexpensive- and can use them for so many projects- and the glue washes away- so they are handy.
wonderunder is one of the fusables that you really have to follow the instructions for usage for it to hold- if the iron temp isn't right- or you don't hold it for the right amount of time...kind of (picky) stuff. are you hand appliqueing ? or machine?
if machine- small pins will take care of small areas that have just come loose-just make sure to remove them before running over them