Rodent ulcers, can anyone help?

My 7 month old persian kitten has Rodent ulcers and has had them since the end of December. I have had her to the vets and he has given steriods twice now. Advised me to get a felliway diffuser and also now put her on some chill pills calls zylkene, she still has them. I think that my 10 month old maine coon is picking on her, I am thinking that I may be faced with re-homing one of my gorgeous cats but want to try everything first. Can anyone help me? I am keeping the cats apart when I am not here but that isn't good either as I got them for company for each other. Please help if you can. Many thanks, Leah