are they slipping at the rear? my response is based on the drivetrain not being worn out....however, if it is an old bike it may well be; best checked at a local bike shop. Most problems however are due to cable tension/quality. If the bike is slow to change up the block, i.e - to change to an easier gear, then the cable is probably too slack. There is a barrel adjuster on either the mech, the shifter, or both, winding that out slightly may fix your problem. if it is slow to move down the block i.e to a harder gear, then the cable could be too tight, therefore wind the adjuster in. Or, it could be due to the cable being worn and sticking in the outer cable, try feeding oil down the cable into the outer...if that doesnt work its time for new cables! not an expensive option! these are just 2 basic ideas, it could be a multitude of factors...worn mech, bent hanger, worn dirvetrain etc....best taken to a local bike shop!

I have the same problem with my daughters bike... anyone out there with advice please ????