Send us a little more detail on the engine, year, etc.., and some pics if you can.
The length should not matter that much, as it does not take that much arm action to pump fuel.
Here are a few things to try:
Make sure that you have it in right side up. Some models are different.
Try bumping the engine over a little and see if it will go in easier against a different part of the cam, and
Measure the length of the arm from the gasket surface, and stick a dowel in the opening and see if it will go in to the same distance, You may need to grind off a little of the end of the arm.