First advice is to not print the whole order until you have succesfully adhered the foil to at least one test print. We even check adhesion throughout the run, as 500 pieces that won't foil right ain't gonna be fun.
So assuming your ink deposit is all good. You haven't mentioned pressure, the third piece of the equation and the hardest to quantify. We do 20 seconds at 350 or so with a medium pressure. Medium to me means some stiff resistance to closing the press completely, but not having to get agro on the thing to shut it. Different designs will require slight variations. If the foil is really hard to pull you are too hot heavy or long. Thats what she said.
Test test test, you will get it. Be sure you have at least an inch or two or more of wiggle room from the size of the design to the platen.
Lastly, we have gotten foil to stick to just about any plastisol, but have been using high density clear tinted to the foil color for awhile, just for it's printing characteristics.
