hey, i think thats usually over watering, or less often under watering. do you let the plants dry out before watering again?
or do they look burned at the tips aswell?
pictures will help id say anyway if possible.
theres a sticky in the sick plants section that shows all the causes for common problems i think.
Depending on how old they are, what medium they are, what nutes are you giving them? Overnuting will cause downward curling. If the leaves are dark green and/or have burning on the tips then it is most likely overnuting.