How many years have you been working in clinical research?
Right now the economy is very bad. Unemployment is very high (~10%), many of those are experienced clinical research professionals. I think that if you have only have 1-3 years of experience, you will find it very difficult to find a job as citizen, therefore it is much more difficult if the hiring manager needs to do the extra work of getting you the H1B visa. I used to work at a small company and they didn't have a lot of resources so typically they wouldn't even consider those needing a Visa because it's a lot of work for the company.
I think that you should certainly keep applying for jobs, but I wouldn't concentrate on trying to get a job abroad right now. You are very limited in this poor economic climate.