Trouble getting tailer ball into receiver

[ccm]

Hi,
I went to hook up my rig yesterday and had a problem I have never experienced. I could not get the trailer ball to fit into the receiver. I put a good coating of grease on the ball and tried again. Still no luck. Checked to see if anything was loose or odd looking under the receiver but all looked good. After about 6 tries I got the ball into the socket by starting the insertion with the ball at the front most part of the socket. Still really tight fit but was able to get it to settle in. I have pulled boat trailers for over 40 years and never experienced this problem.

Any suggestions as to what would cause this problem and a solution to prevent it happening the next time I use the trailer.

Thanks, everyone, for your advice.