Since lever harp strings are not standardized as far as diameter of string are concerned, did you use the size string the manufacturer has indicated for the particular harp you are playing?
Also, harps can be tricky in this way; after you have determined that you got the right diameter string installed in the right place, be sure you have the string tuned to the correct octave.
If you still aren’t happy with the sound, take it to the place you are renting it from, your teacher or contact a technician.