When your children are suddenly forced to live between two homes because of a divorce, they are likely going to grow closer to the parent they are with most of the time. This can leave the relationship with the other child lacking in some of the same elements that are present in the one with the custodial parent. It is up to both parents to try to rectify the situation.
If you are the custodial parent, you must ensure that you aren't trying to keep the relationship limited. You might do this unconsciously, but you have to try to catch those actions. Instead, encourage your child to have a meaningful relationship with your ex. Unless that adult was abusive toward the child or has hazardous behaviors, there aren't any reasons why you shouldn't try to help that relationship along.
Remember that your ex might be feeling a bit down because of the lack of relationship. They might even be questioning their parenting abilities. Your actions toward them can go a long way in trying to show them that you think they are a good parent. Remember that just because your marriage ended doesn't mean that your ex is a bad parent.
The parenting plan can also play a part in the noncustodial parent's relationship with the children. Ensure that there is ample time for them to build that relationship. Even when the child is at your home, phone calls, video chats and other communication might prove to be a positive way that they can keep in touch. Terms to help this happen might even be included in the parenting plan.