Java Programming

I am using a TreeSet in a class that extends AbstractListModel. It seems that the TreeSet.remove() and TreeSet.contains() is not finding the object. If I run the code below, the element is not found (sometimes).

public boolean removeElement(VariableInfo element){
        boolean removed = model.remove(element);
        if(removed){
            fireContentsChanged(this, 0, getSize());
        }
        return removed;
}

If I run this code, however, the element is found. (The code below will fail if the uncommented line is used.) Any ideas why this is happening? I don't want to use the code below because it requires iterating over all the elements in the list.

public boolean removeElement(VariableInfo element){
        for(VariableInfo v : model){
            if(v.equals(element)){
                return model.remove(v);
                //return model.remove(element); //will also fail if using this line instead of previous one
            }
        }
        return false;
}