Just make sure the line is clear of fuel (or it can exit) -- otherwise you trying to compress the liquid. If you have a fuel shut-off at the tank, then make sure it's open. (It's just an attempt to find an explanation for why it won't slide-on. It usually is an issue getting it OFF not ON ;^)))) )

I'd be careful with heat, you really shouldn't need it. As Todd pointed-out, a little diesel should lub the fitting enough to slide it on. You could use a LITTLE clean engine oil -- or maybe WD40 to help slide it on.