As my dad always told me: "Don't force it, get a bigger hammer!"
Sometimes the prop pops off with just a little encouragement. Other times, you need to apply all the force that the puller is capable of exerting.
How big it the wrench you are using on the puller? If you are using a 3/8 socket, there is no way you would be able to get enough leverage to do the job. I recommend using a 1/2" breaker bar, and then slide a section of pipe over the bar handle to get some additional leverage. You will need to jam a 2/4 or similar between the prop blade and the hull to keep the prop from turning.
If all else fails, try tapping on the prop with a hammer while you are torquing on the nut. DO NOT APPLY HEAT, you will ruin the strut bearing.
WARNING!! Pad the frame of the trailer, wear a helmet and elbow pads, or do something to keep from killing yourself when the prop does pop loose. When it comes loose, it will come loose all at once and since you will be straining hard against the breaker bar you will tend to go flying too.