You can modify child support after divorce in Maryland if there has been a material change in circumstance since entry of the Order setting child support. The applicable MD statute is Section 12-104 of the Family Law Article which provides
Modification of child support
(a) The court may modify a child support award subsequent to the filing of a motion for modification and upon a showing of a material change of circumstance.
Modification not retroactive
(b) The court may not retroactively modify a child support award prior to the date of the filing of the motion for modification.
Material means 1) relevant to child support and 2) of sufficient magnitude to justify a change. There is no bright-line cut-off stating any required minimum change in support. Obviously, the process requires time, effort and money, so minor increases or decreases may not be worth pursuing.