You need to explain more clearly what the problem is.
Are these tanks ordered to move by trigger or manually by you? If by trigger are they owned by a Computer with active AI? Is the problem that their orders are cleared (siege mode, stun etc)? Is it that they are blocked by other units when trying to get in range so the move order time expires?
Trigger owned by players. After the unsiege, they don't move to the marker point.
That is because sieging up removes all move orders as siege tanks in siege mode cannot move. You need to re-issue them after they have fully un-sieged. Easiest solution is that MaskedImposter suggests where you re-order them periodically, possibly staggering to avoid spike resource consumption.