Physiotherapist Richard Watterston replies: Weak ankles are often caused by people going over on them and straining the ligaments on the outside of the joint. If this injury is left to heal on its own with no rehabilitation, there can be a loss of proprioception (this is the ability to detect and adjust to changes in balance) and this can cause pain and injury.
To improve this, practice balancing skills on a wobble board to retrain the reactions that will reduce the chances of straining the ligaments further.