[msaharia]

They couldn’t come because of any number of reasons: there wasn’t enough time to apply for visa because you issued the letter 1 month before the meeting, didn’t get visa, didn’t get visa on time, visa got approved but passport didn’t arrive on time etc. You also can’t get reimbursed in India without showing actual receipts of conference, hotel etc.

You seem to think that your letter is a guarantee of visa. It’s not.

Source: I’m an academic with both US and Indian experience.