Everyone has made great suggestions:
Have the vet check for a UTI
Larger boxes
More frequent cleaning
Uncovered boxes
Cat attract urine
Change her environment so there are fewer, or no other cats
The one thing I'll add that hasn't been suggested yet is plain paper - don't shred it, just lay it in the bottom of the litterbox and see what happens. I hope with all these suggestions you find the one(s) that fix the problem for her!!
Have the vet check for a UTI
Larger boxes
More frequent cleaning
Uncovered boxes
Cat attract urine
Change her environment so there are fewer, or no other cats
The one thing I'll add that hasn't been suggested yet is plain paper - don't shred it, just lay it in the bottom of the litterbox and see what happens. I hope with all these suggestions you find the one(s) that fix the problem for her!!