A weak ankle
Some people feel that their ankles are 'weak'. This might be a recent problem or a lifelong issue. Perhaps it keeps going over easily since a bad sprain. This is one example of an ankle not getting better after an injury. There is a variety of causes each of which has its own treatment. Where the problem is long standing, we look for excessively stretchy ligaments throughout the body (hypermobility) or for abnormalities of the way the bones are structured. If the heel stands tilted in and a the arch is deep (cavo-varus foot) it is not hard to understand how such an ankle might begin twisting easily with minor provocation. Progressive damage to the ligaments and tendons supporting that ankle will cause even more twisting episodes as the joint loses the control and support it requires.
Rarely, a lifelong sensation of weakness is because there are abnormal joins between bones that should be free to move (tarsal coalition).
Ankles can feel weak without actually going over on them frequently. Once a true "structural" problem has been excluded, a good programme of physiotherapy might restore the strength and control. However, if there are repetitive sprains with pain and swelling between episodes, the problem should be treated to reduce the chances of worening internal damage to the ankle joint.