Make sure theres nothing obstructing the rod from fully seating. There are different types of m/c, some have the channel for the push rod clip, some don't. Reach down into the shaft with a small piece of wire to make sure a channel even exists. If it does, are you using the same rod that came with that m/c? The rod lengths and end are different. It should only take a good shove to seat the push rod. If it doesn't, something else is going on. Here's an example, both from ford, two completely diff rods.