Occasionally you will jump on a list, hoping to be among the first to order and, therefore, be assured a good fill rate, only to receive, when the shipment finally arrives, a fraction of what you ordered. Or you will watch my lists and discover that between one week's list and the next, 75% of the inventory of one wholesaler disappeared.
One reason this happens is your competition shops the warehouses. Usually the effect is not so obvious, but if one warehouse is visited by two or 3 large customers in a 2 or 3 week period, it can play havoc with orders and inventory. About 4 years ago I was present when one customer made a deal with the wholesaler to take virtually all of their inventory. This was one case where the haggling was hugely successful for both parties. That wholesaler's list did not recover for six months. Most orders placed at the time had to be canceled.
For most buyers, warehouse visits are out of the question unless they have a wholesaler within a day's drive or if there is a trade show the buyer attends which is located near a warehouse. Still, if you ever get the chance, take it. You will be taken more seriously by the wholesaler, find wonderful surprises everywhere you look, and get a feel for how the supplier treats their inventory. It also does not hurt to get a better understanding of what it takes to do that business, a daunting challenge that few bookstore or internet buyers appreciate. Maybe most important, use the opportunity to get to know the financial manager and talk about terms and what they need from you to improve them. This can make a big difference down the road.
(Shameless pitch time: Please let me set up a warehouse visit for you with one of my lines. I can figure the best times to visit and tell you if someone you're thinking of visiting is running lean at the moment. A big part of a successful warehouse visit is not to come in too close to a large buyer's visit or after a major sales push.)
Another reason you might be missing titles is that there are those among your competition who really do jump on the lists. I have customers who buy regularly, broadly, and order within hours of my sending the lists, even so far as sending me an email within minutes saying, "I see you have such and such by so and so, please put in an order for me for 50 copies, I will get the rest of my order to you later today." And I might receive this email at 10:00pm on a Saturday. Yes, we're nuts, that’s why we're in books.
So, no need to be extreme, but do pay attention, and think about visiting a remainder wholesaler's warehouse or two. I'll tell them you're on your way.