If you share parenting duties with a person you are not in a relationship with, there is always the chance that person could suddenly refuse to allow you to see your children. While this is not necessarily a legal move under Florida law, it still happens.
According to The Spruce, parents may keep children away from their other parent for many reasons, but most visitation denial reasons are not going to stand up in court. For example, child support and visitation are seen as two separate issues by the courts, so a parent cannot stop visitation due to nonpayment of child support. A child custody arrangement is a court order, so when it is not abided by, it is a violation of the court order. Unless there is a very strong and compelling reason for your ex to not let you see your children, you can take it to court and have a judge step in.
However, before going that route, you may want to try to talk to your ex first. Find out why he or she is not letting you see the kids and if the situation can be remedied. Trying to work things out without involving the courts can be easier for everyone, especially the children.
If this does not work, then you can call law enforcement to get documentation of the issue. Officers cannot do anything but file a report, though. You should try to document as much as possible about the times you are refused visitation. This can help in court. This information is for education and not intended as legal advice.