An issue is never closed in any absolute way. Most councils have standing orders that include a stipulation that the same matter cannot be raised again within six months. However, this is not a statutory requirement, and it is thus possible for a council to vote to suspend that particular standing order and then to revisit a matter at any time. The standing order exists only to prevent a minority of councillors from repeatedly raising the same issue.
If there is an unresolved issue, then it could be considered by the council either because the clerk considers it necessary to put it on the agenda in the interests of the council or because a councillor requests an agenda item on the issue. Provided the request is made in the way prescribed by standing orders, the clerk must include the item in the agenda, and the council will then need to consider it. However, the council is not forced to decide on it, since it is always possible for the council to vote to move on to the next item without making any decision. Once again, it all depends on the majority view of the council, as is reasonable in a democratic system.