Regarding stock levels, they were talking about this in the Brexit deal and that many manufacturers don't stockpile parts or stock. Everything arrives in as required and then out the door again. So if we are going to be using middle men wholesalers to buy our stock, they have a duty to do their 'one job' and stockpile goods. We may as well buy from China direct.
If I'm out of stock, as a retailer, my customer goes elsewhere, whereas the wholesaler knows that we will probably wait and buy later. They don't loose out as we do. And who has the cash flow and warehouse space to stockpile mugs for 6-9 weeks of supply?
But surely that also relates to a supplier.
You operate as a just in time supplier to your customer (otherwise you would have plenty of stock) so why is it that a supplier can't operate in the same way. As you say you may as well buy from China, but if you use that reasoning then look at the costs v stock. Firstly your saving will be nominal (I would be surprised if it is more than 15p per unit) and you will be required to purchase thousands of whatever item it is which is something that you (or me tbh) wants to do. I like just in time delivery but I also appreciate the risk involved. I just don't see it as a supplier fault.
You have used mugs as an example and it is common knowledge that for most suppliers there is pennies in the profit for mugs. If we are not prepared to pay a higher cost for the unit why should they hold thousands of mugs on the shelf in the hope that you have a good month and sell more than usual.
My issue isn't so much that they don't have it, it's that they run out and don't tell you until you chase the delivery. You're then told stock is coming in and it just gets delayed. I appreciate that's not always the suppliers fault and it's hard for them to work with manufacturers overseas but that's their problem, we have enough of our own 
If there is a thread about supplier communication I am 100% right behind you

USING: Whatever it takes to get the job done...