I think Mulrine is absolutely right - discuss a plan B and possibly a plan C with your supervisors before you start recruiting, you will probably have to write about this in your ethics application as well. When you mentioned the groups, did you mean that you need 70 participants from each group, totalling 140 participants? What is your methodology?
With 'hard to reach' groups, snowball sampling can be an option, maybe look into this? I am in the field and recruiting at the moment and although some people have been very forthcoming, I want some variation in my sample and so I am taking advantage of participants being recommended to me. I only need about 15 participants though, so it sounds as if you have a different sort of problem - some more info about your project might be useful :)