Over the last several weeks, a number of distributors and publishers have reported delays in Amazon picking up orders from their docks to deliver to Amazon distribution centers. Companies have attributed the delays to a number of issues, including the nationwide shortage of truck drivers, Amazon’s shifting of resources to Prime Day (which is set for July), and changes in the online retailer’s supply chain.

One company executive, speaking on the condition of anonymity, told PW that, last week, Amazon had twice said that trucks were on the way to pick up its books, only to have the trucking company cancel. (Amazon uses several different trucking firms to go on what are called “milk runs” in the industry, where they pick up orders from suppliers.)

Other company execs told similar stories–one said they sent a purchase order to Amazon, the company assigned a truck to pick up the order, but the truck never appeared. At the start of last week, the situation was serious enough that a few publishers said some of their books were showing up on the Amazon website as available to ship within one or two days, or temporarily out-of-stock.

Company officials said they have been in touch with Amazon and were told that the issues were being worked on. While not addressing the causes of the problem, an Amazon source told PW the online retailer had “recently rebalanced our capacity to ensure the speedy pick-up of items.” By the end of last week one company exec said trucks from Amazon had arrived to pick up a large order, but another said late Friday they were still waiting for confirmation when a truck would arrive.

While an executive at one company observed that they ran into similar problems last year before Prime Day, others suggested that Amazon’s rapid expansion is making it difficult for the company to maintain its supply chain as efficiently as it has in the past. One publisher said that given Amazon’s size, “hiccups” in fulfilling orders are to be expected. He then added: “You just hope the hiccup doesn’t effect your book.”