Why? This happened after the latest patch? Never did it happen before it.

It was known to happen before the patch as well.

When you go to a trader, follow these steps as a work around.

1. Enter the barter screen and exit again.
2. Make a save standing next to the trader.
3. Shutdown the game and restart.
4. Load your save.

There is still a minimum time limit applicable to traders restocking.

Thanks for the info. The trick worked!

That works for me also when my companion won't come inside.

All this does for me is refresh their caps. They still have everything I've ever sold them...That goes for all traders. That could be why my save file is exceeding 8MBs.