There are two main things which prevent the head from coming off.
If the head seems to be "glued" down to the barrel, and won't budge, then it probably IS glued to the barrel.
For this, you can take a very thin paint scraper or a thin blade knife, and very carefully tap it into the head gasket itself from the side, with a mallet. By tapping into the head gasket joint, it will break the seal of the head gasket sealing compound which sometimes is in there, and the head will release. You have to work the blade all the way around the whole engine, tapping it in to the head joint from all around, a little at a time. Eventually the head gasket will be destroyed from this process, and won't hold on anymore. Then the head will lift off.
You want to use a very thin blade, and try not to harm the aluminum surface of the head. You may get some scratches, but that's not any real problem. Just avoid gouging it. It works, and I have done it many times.
The other common issue is that it gets stuck on the head studs. This is usually because the washers used under the head nuts get squished when they got tightened-down, and now they bind against the studs, making it hard to get off.
You can either remove all the studs, or you can squirt some penetrating oil down the stud holes, and wiggle the head a little at a time, working it up the studs. Or, maybe only some of the studs will come out easily, and then you can take them out, and just wiggle it up the remaining studs.
Put new hardened washers in there stud holes when re-assembling, AFTER getting the old washers out first.
You can get the head off. It might take some effort and time, but between these two methods, I have never failed to get a head off.