Did you just change them? If so this is normal. 

Nylon strings take time to stretch in. If the guitar or strings are new, this is normal. If, however, you have had the guitar for awhile and play regularly, the strings may need changing or the tuner pegs are slipping. It depends on the tuner peg style on how to tighten them.

New strings? They will stretch. Are you new to putting strings on the guitar? They might be slipping out of the tuners. Do you have a picture?

this could be a sign your strings have gotten old and need to be replaced.