What do I do if some of the iron-on doesn’t stick after I put heat on it?

Be sure to use the recommended settings for the iron-on and base material you're using.
Ensure you're using a firm, even surface. Ironing boards and plastic surfaces have some give in them that don't provide the firm, even surface needed for successful results.
Re-touch the front and back of material with heat for an additional 10-15 seconds.