Loose frets??
A guitar with loose frets can be a problem when it comes to getting the neck playing right, if they're unstable they will need solidifying before I can do a fret dress on them!
How can frets become loose?
Frets can come loose and start to lift up in the fret slots cut into the fingerboard, this happens when the timber of the fingerboard dries up. Frets have a small barb which is designed to help the fret stick in the fret slot.

Most people "oil" their fingerboard with oils derived from trees such as Lemon oil, teak oil or Almond oil to stop their fingerboards drying up. I soak almond oil into my rosewood fretboards, it's very nourishing. If the frets have been installed poorly in the factory then it's more likely that the frets will pull out of their slots.
Here's a fretboard I've oiled only half way up the neck!

If your guitar is found to have loose frets, this needs to be sorted before the frets can be levelled. This isn't too hard to sort out luckily. For a small extra charge, I put extra runny cyanoacrylate glue down into the fret slots to solidify them, clean up the excess glue and then continue with the fret dress on some stabilised frets!