I've only just realised a similar thing. I changed my address to my parent's before moving country and before I paid the remainder of my deposit (early April).
SeeTickets sent it to my old address mid May and RM have returned to sender according to the tracking link. I have emailed SeeTickets and am praying that they can sort this out. I should have checked the tracking link earlier, but also I don't understand why they would send it to an old address.