No, the percentage of alcohol should not make this big a difference. I'm picturing that you made the common bottle, alcohol/water, straw type thermometer. I suspect the the most common failure mode of this device is in the stopper. The clear straw has to go through a hole in a stopper in the top of the bottle, and there can be absolutely no air leaks in the stopper, either around the edges or where the straw comes through.
If the top of the bottle has any air leaks, then as the water/alcohol mixture heats up and expands, it will simple rise slightly - imperceptibly - in the bottle and NOT force any liquid up the straw.
If, on the other hand, the stopper/straw assembly is air tight, then as the liquid expands slightly, the trapped air in the top of the bottle will exert pressure on the liquid (since the expanding liquid will try to compress the air and the air will push back) and the only place the liquid can expand into is the straw. Since the top of the straw is out in the open, there won't be any increased downward pressure on the liquid in the straw resulting from the expansion.
So...my guess is, if you basically assembled the thermometer correctly, you have an air leak around the stopper/cork at the top. You can try using modeling clay to seal it all up air tight.