Having dealt with Spencer since the late 90's here is my advice:
He's a one man show with tens of thousands of pages in inventory at any given time. If you attempt to purchase items during his famous year-end sales you are in good company. From past discussions with Spencer I know he receives orders from several hundred collectors and those orders amount to several thousand pages. He takes the orders in the order within which they are received. This is where the delay in response will occur. The reason being, Spencer is an artist's representative, he's not a dealer. The items he sells are not his property. That means various artists may or may not have some or all of the listed inventory in THEIR possession and their availability must be confirmed on a piece by piece basis. Some of these artists are local to him but most are scattered around the globe. For the international artists he tries to keep as much of their inventory on hand as possible, but that is up to the artist to decide.
Some reps have several employees, and others mandate all their artwork be kept in house. That may speed their response times. Many other reps don't have 50,000-100,000 pages in inventory at all times which also helps speed response times.
Spencer goes through each order as I noted, confirming availability, invoicing and confirming the order, buyer by buyer. The dramatic nature of some of the discounts during his holiday sales are such that a feeding frenzy occurs with several potential buyers for each piece and an enormous order volume. Those discounts, however, are worth the patience they demand from the potential buyer.
Aside from his annual sales Spencer has also begun, in the last few years, traveling to various conventions across the country and selling artwork there. If you happen to place an order while he's on the road he may not be able to get back to you substantively until he returns, runs inventory, and can confirm the piece(s) in question are still available.
If you find yourself unable to get a response to an order I'd wager it's a situation of high order volume during the sale, or artwork being in the hands of the artist and its availability needing to be confirmed, or the artwork was part of the inventory taken to a convention and again its availability needing to be confirmed.
So I wouldn't assume you're getting "roadblocked" by the rep in this situation, there are myriad potential reasons for any delay.
When it comes to Spencer I always know he's someone I can trust explicitly in every single dealing I've ever had with him and that, in and of itself, in invaluable in this hobby.