It happens all the time that a peg is so tight that it can hardly be turned or they loosen and the tuning is ruined.

A well-functioning peg must fulfill several criteria:

• It must fit properly. The holes of the peg box must be evenly formed and provide a perfect fit for the peg.
• A peg must remain adjustable. As the peg soap is hygroscopic (it attracts moisture), the peg reacts to differences in humidity with different operating characteristics.
• A peg needs care. A good idea is to purchase some pure curd soap (preferably old) and also regularly white chalk it.
• Remove one string and carefully clean the peg with some alcohol but be sure to avoid any contact with the instrument's varnish.
• Rub the peg with the soap.
• Turn the peg several times in the peg hole.
• The thread of the peg box should be clearly visible on the peg.
• If this is not the case, the peg does not fit properly and should be re-fitted by a violin maker.
• Repeat the soap rubbing and turning so often that the peg works smoothly and with ease.
• Now apply the chalk to the contact surface of the peg and turn it repeatedly. Do this until the peg turns firmly but without bucking. Replace the string while paying attention to the incline of the bridge and proceed with the next peg in the same manner.

Detailed information and photos can be found here.