Mine ran OK from Fred so either he put ball bearings that were slightly too large or there is grit in there. My experience is that they don’t clean them so that is possible although I’ve not had one which felt gritty.
Fred is usually very helpful so I would mention it to him as I’m sure he would appreciate the feedback and he may offer a few pounds back for the time you will spend taking them apart and cleaning them. Unless you think they are damaged I would spend a bit of time getting to know how they work inside.

There are lots of videos showing how it is done. Make sure when you remove the nut from the screw you catch all the ball bearings which fall out. There are also return guides (orange plastic) which you can also remove but note how they go back.

I don’t have a recommendation for what cleaning fluid to use, some kind of mild degreaser on a cloth.

To refill the ballscrew tracks put a dot of grease on the end of a small implement and use it to hold the ball in place whilst you lower it into the track inside the nut. Use more grease to hold it as required. Repeat until full then carefully insert the keeper tube to hold the balls in place then screw it back into the ballscrew ensuring it is the right way around! It’s doable but fiddly.

Here is my video showing it around 6:00 mins in:
https://youtu.be/yJBE-fxVh8Y