large "growth" on throat area

Anyone have a horse with a seroma that won't re-absorb? I have one with one on her throat. NO infection. I finally drained it today and she was happy about it. It did re-absorb 2 summers ago but now it won't. Will be visiting a vet to find out why. Doesn't hurt or bother her. It doesn't worry me - just want to know how to get it to re-absorb or why it won't.