In Maryland, a party can only seek a modification of custody if there is a material change in circumstances.
There are a number of cases that give an idea as to what a material change is. Generally speaking it is one or a combination of the following: a significant change that occurred since the entry of the initial order; something material that was unknown at the time the initial order was passed has since come to light; and/or something since the entry of the Order is adversely impacting or could adversely impact the welfare of the child(ren). One circumstance that has been ruled a material change in Maryland is the relocation of a parent. Braun v. Headley, 131 Md. App. 588 (2000). When facing relocation of a parent, the court may consider such factors as a potential change in visitation schedule, child's school change, the moving parent's reason for the move, and the ability of the parents to both maintain a relationship with the child after the move when deciding whether or not the move will constitute a material change in circumstances. More often than not, a relocation of a parent is a material change in circumstance.
Assuming the parent seeking to modify the existing Order establishes a material change in circumstance and the Court so finds, the second prong of the two prong test is what is in the best interest of the minor child(ren). In deciding what is in the best interest of the child(ren) the court considers a host of factors. Some of those factors include the fitness of the parents, the desires and agreements of the parents, the potential to maintain family relationships, the child's preference if he or she is at a sufficient age, opportunities affecting the child, the age and health of the child, and the residence of the parents and opportunity for visitation. Montgomery County v. Sanders, 38 Md. App. 406 (1997). It is important to keep in mind that even with a finding of a material change in circumstance, a modification of the custody order still must be determined by the court to be in the child(ren)'s best interest.
Also, you can file a contempt motion any time the other party violates a term or terms of the court's order.