This answer is from last year, but is still valid for 2017 returns:
https://ttlc.intuit.com/questions/3843655-i-received-a-letter-from-the-irs-they-are-holding-my-refun...
I would contact the IRS to see what the hold is. Only they can tell your what is going on.You can call 1-800-829-1040. Customer service representatives are available Monday through Friday, 7 a.m. to 7 p.m. local time, unless otherwise noted.