You are probably "stuck" in some of their internal checks, and only they know exactly what is happening. Are those coins that you bought on their platform or did you get them in some transaction? It is possible that they investigate the money/account with which you bought them, or if it is another option, that you received coins that they consider tainted for some reason.

Also, are you 100% sure you used a legitimate site? It seems to me that this is not the case, because "support" would probably ask for an additional payment to enable you to withdraw, but everything should be taken into account.