What kind of fusible interfacing are you using? Since for a playmat it may be thin and stiff, like Decor Bond 809 or 808 or maybe fusible fleece? At any rate, what I do is lay the fusible on my ironing board rough fusible glue side up, then place my fabric wrong side down on top of the fusible. Smooth out with my hands and then press with my iron. The key is to press not iron, lift the iron up to move, not slide. Leave the iron in place long enough to melt the glue and adhere to the fabric.....there should be directions with the fusible...usually a couple of minutes each time you put the iron down. A large piece like a playmat will take some time to fuse.
Hope this helps.