For example, if your Beastmaster job is level 40 and is set as a sub for a level 75 job, a monster that checks as Easy Prey will probably be very difficult to Charm. However, if your Beastmaster job is level 75, and if you set it as a support job to another level 75 job, an Easy Prey may be Charmed with ease.
/ja Charm <stnpc>
This varies with both your level and the difference between your level and the mob's. Particularly when charming Tough mobs, the exact level of the mob makes a significant difference (for example, charm lasts significantly longer on a level 76 mob than on a level 77).
A monster will only uncharm immediately or very fast, if, after level sync it would check as tough or higher to the level that you are syncing down to. If, however, the monster would check as even match or below then it will stay charmed, furthermore, it will stay charmed for as long as it would have at your original level when you charmed it i.e. if you charm a monster at a level when it checks as too weak and then level sync down to a level when it will check as even match then it will stay charmed for 30 minutes as is how long a too weak monster would stay charmed.