Reem,
We are also experiencing difficulty with the plant category feature. We have one plant list that is designed to separate the plants into their particular 'plant categories.' However when i recalculate the plant list there is usually a few plants that have been misplaced (perennials listed as trees, shrubs listed as perennials, etc). the only solution i have found is to edit the plant catalog, coming all the way back up the chain of command to the plant reference database (where the plant category resides). Everytime the proper plant category is listed and i simply have to 'ok' the creation of the particular plant all over again. this usual solves the problem but i find it rather problematic that i constantly have to double and sometimes triple check the plant list.