This one has everybody scratching their head... here's a layout of our plumbing - water comes in house, laundry is the next "downstream" fixture, then guest bath, then kitchen, then master sink, and finally master shower is the last "downstream" fixture. Here's the issue - hot water never makes it to the last "downstream" fixture but it does make it to every upstream one. Example - guest shower is cold until you turn on any downstream fixture (the kitchen sink, master sink, or master shower). Similarly, master sink is cold until you turn on the downstream fixture (master shower). Any ideas??

If you have single handle faucets in the sinks or showers, or a single lever washer box, hot water can be circulating through the faucets. I would start by shutting valves off to each fixture individually and see if one or more of them being shut off solves the problem. If it does, just replace the cartridge in those faucets and you should be all set. This of course is a shot in the dark without being there to test for myself, but I have seen bleeding through of single handle cartridges cause problems similar to yours.