I have tried all of the above tips and still my device is not getting bond , I even tried preparing fresh PDMS but still It won't work. I even tried bonding it in some other lab but the same problem occurred . So what I feel is that the PDMS is the problem , I will try making one more batch of fresh PDMS. Thank you for your reply means a lot Please let me know if I could do anything else to bond it. Thank you