Inadequate inventory seems to be the problem with your store. You ship items in stock quickly, but unless the customer receives all of the order, the project can not be started and completed. This makes your store useless to the customer. I suggest, on long lead time items, offering substitution products, such as a different brand, drip tube 6" on center instead of 12". Anything that will allow a project to move forward. On line purchases are more convenient and cheaper than local supply houses but not worth the wait if products can not be obtained in a timely manner.