Sixteen thousand were sent out to season ticket holders, members and others but so far only a couple of thousand have been used according to Ticket Office Manager Stephen Ebanks.
The voucher allows someone to bring a friend for a home game on a Saturday or a night match for only £10 and according to the voucher, the offer ends on Saturday February 12th 2011.
Mr Ebanks told the club website: "One of the perks of buying a season ticket this year was a couple of vouchers to pass on to friends or family to come along for £10.
"But, as of yet, we haven't seen many taken up. I would imagine that many supporters have put their season ticket information away in a drawer and forgotten about it.
"This is really just a message to them to dig out the information and to make the most of those vouchers. There are only seven Saturday games to go this season and just a couple of weekday games so my advice is to have a look at the fixture list, have word with your friends and don't miss the opportunity."