There may be a couple different reasons that the credit was disallowed. You should be getting a notice in the mail from the state stating the reason for the adjustment shortly.
The first reason may be due to backup documentation. According to the state, you are required to include backup documentation for the repayment with your filed tax return. Did you submit this with your return? Administrative Release 40 states, The required documentation, which should be included with the Maryland income tax return filed for the year the repayment is made, is a copy of the document which states that the taxpayer must make the repayment, and a copy of the document which provides evidence that the repayment was actually made.
Secondly, the credit is only considered valid if you appeared to have unrestricted right to the income in the prior year and repayment was made because you did not actually have that right. However, if it is found that you did have full rights to the income and then later became responsible for paying it back due to subsequent events, then the credit will be disallowed.