They can be tricky to get started sometimes. The weight of the rod tends to make it want to tip down.
Mine has been pretty corroded at times, but I have always managed to get it in.
Doesn't help with the various components in the way either.
I always start mine by hand, looking at it to make sure it is straight in and not cross threaded. You can tell by looking at it whether it is in straight or not. If you cross thread it and try cranking it in, you will mess up the threads. Then the above mentioned tap to chase the threads will become essential.
I have always used a stiff wire brush on the rod threads, and a small toothbrush sized wire brush on the tank threads.
Never had a problem that required a tap. If it comes out, it will go back in.