Adding a 4th Transponder Key

I'm new here and am I glad I found this forum.

I bought a used 2005 Galant and got two registered keys as well as the valet key. I bought two blank transponder keys on eBay and had them cut by my local locksmith.

I followed the Users Manual's instructions to add additional keys and was able to add the first new transponder key, just like the manual says to do. However, when I tried to add the next key, it doesn't seem to program correctly.

Since it's a used car, could it be at it's limit to the number of keys? After all, I don't know how many keys were added before I got the car. Or is there something else I'm missing when adding this 4th key?

Thanks!

get a scan tool and see if that will help it programm it