Spark Plug not going in all the way?

I was replacing my spark plugs on my 01 Ford Sport Trac when I got to the one closest to the firewall on the drivers side. The plug that was already in was very difficult to get out, but I managed to remove it. When I tried putting the new plug in, it wouldn't catch onto the threads in the engine block. After trying multiple times to get it to catch, the arch on the end of the plug was bent, and I damaged the threads. I went to an automotive store to get a tap and an insert so repair the thread. I believe I repaired the thread in the block, but plugs still won't catch tighten all the way down. It almost seems like something's blocking it from going in all the way. It's been 125k miles since the last plugs had been installed, and the truck has idled roughly for sometimes now. Any suggestions on what could be wrong or how to fix it?

So you were able to rethread the block but you still can't get the plug in? Check the diameter of the spark plug you're trying to put in. One of the other plugins you installed to see if that one will fit. It almost sounds like you have two different sizes of the same plug or completely different plugs

Make sure Heli-coil or whatever brand thread-sert you used is flush or below edge of spark plug opening, and bottom tang got knocked off. Ensure that it didn't go into plug hole. Plug should thread in easily, no excess force required. If not, start over. Apply anti-seize compound to threads when finished to avoid this problem in the future.

You had the right idea a spark plug hole repair KIT, but I guess you got that gross threaded to now try it again with lock tight red is still don't work your screwed, have to replace head or take head to a machine shop to get repaired right

I was able to get the plug in. I just removed the left front tire, and the skirts underneath the wheel well and got a much better angle. The plug practically went in no problem then. I appreciate the answers guys!