Several times I have not been able to easily get my tow bar to lock after it is attached to the tow vehicle. I use the Excalibar tow bar. I really like it. I usually connect the bar, back the tow vehicle until the bar locks with a click. This usually only locks one side. I count on pulling the coach forward to pull the other side into the locked position.
I tried using a lubricant on the slider bar (rod), and that worked for a while but then stopped working. I found that it worked much better when I used a solvent (WD40) to clean the rod. I spay the rod then wipe it clean. I have found that the bar seems to work much better when clean than when it has a lubricant on it. The lubricant seems to pick up road dirt and the dirt works its way into the lock mechanism. So, if you are having this type of problem, try cleaning rather than lubricating the tow bar rods.
One more thing to check. Be sure to periodically check the bolts in your towed vehicle tow brackets. I recently heard a horror story about an RVer that arrived at his destination to find that his towed vehicle was not with him. The bolts may work loose, so check to make sure all bolts are tight and the tow bar is securely attached to the RV.