Android tablets only connected to 2 PCs on network


Feb 8, 2014
So, here's the situation:
In my house there are 5 PCs (all running Windows 7, Home Premium x64) connected to a router, 4 via ethernet cables and 1 via wireless adapter. All shared folders can be accessed by Everyone.
No problems.

Then we get 2 tablets (Google Nexus 7 & Samsung Galaxy Note 10.1). At first both tablets could connect to all PCs but one. The naughty PC was giving all the "SMB server is off" and "Blocked by firewall" messages from ES File Explorer.
Without changing anything another PC started giving both tablets the errors on ESFE. I had been playing videos a few hours earlier and now suddenly, I get errors. Within an hour, the third PC gives the errors.

I reset all the naughty PCs one-by-one and after about 15 minutes, they were accessible on ESFE again...but not for long. After a day of successful ESFE use by both tablets, they all start giving the errors (in the same order too).

Only two PCs are always accessible: the one using the adapter and the one that is the Network Host (if that matters, otherwise it's just another ethernet cabled one).

* Only the tablets have problems accessing the network.
* All network locations are visible but 3 give errors and are inaccessible.
* I have reset both tablets to no effect.
* All PCs are similar builds and have essentially the same programs and nothing that would block/interfere.
* I have tried several file explorers from the app store.
* Neither tablet is rooted and not likely to ever be.
* The order in which the PCs start giving errors is the same (reverse-alphabetical) - don't know if that matters, but I'm looking for any and all clues here!

So, if anyone can help me understand what the deal is with the network and tablets, I'd be super grateful.
Or if the host computer is important, can someone help me change that?

Thanks for reading!