Depends. Do the excuses seem credible? If they do, then it's just a matter of finding a different time that works
If you don't think they're credible, you could try:
• Finding a mutual friend or colleague that they respect and asking them to step in for you
• Reinforcing the importance of the interview to you (if there's nothing in it for them)
• Adding an incentive for the interview, e.g. a gift card, discount on your product, etc
• Asking them to recommend someone else who they think would fit the bill
• Leaving them alone if it starts to seem like you're bothering them. You'll find others to interview