FIL visitor's visa rejected thrice. Help Required!

[shilpa_akella]

My FIl applied in January, March beginning and March end. He is retired. Any help regarding the situation is appreciated. Should we wait for some time before we re apply for his visa. All my husband's siblings are in the US. The consulate did not give any specific reason for rejection. Is there any specific documents that we need to take? questions that we need to prepare? Is it ok to re apply for the 4th time within 6 months?MIL got visa in 2nd attempt.

Any help/information is greatly appreciated