Had the same problem. Here's my setup and solution.
1. Windows 7 Home Premium (upgraded from Vista) with local Administrator account: Homegroup created here. This machine could access Homegroup shares on both other machines.
2. Window 7 Enterprise machine with local Adminstrator account (same account credentials at the Win7 HP machine): Could access Homegroup shares on both other machines.
3. Windows 8.1 with Microsoft account (not a local account): Could access Homegroup shares on the Windows 7 Enterprise machine. Could NOT access shared content on the Windows 7 Home Premium machine even though they were shared to the Homegroup. The Windows 8.1 machine did display the shared libraries under Homegroup, but would not open them. I also shared some folders, but got a permissions error (access denied) when attempting to view them.
SOLUTION:
Rather than creating a local account on the Windows 8.1 machine (which did work, but was not the solution I wanted), I shared my libraries to the Homegroup AND to a specific user (the Windows Account on the Windows 8.1 machine), as follows:
1. On the machine from which you are sharing, select the desired library, folder, or file.
2. Click "Share with" | Specific people... on the toolbar (or right-click).
3. Enter the email address of the Microsoft account (i.e. user@domain.com) and click Add.
4. Select the desired permission level (Read or Read/Write) and click Share.
4. When sharing is ready, verify that the Windows 8.1 machine can access the new share. (It may take a while if sharing many folders/files.)
5. Repeat for other libraries, folders, or files as desired.
This is probably the first time I couldn't find any help on the internet, but finally figured it out on my own! Hope this helps someone else. Still not sure why this problem occurred on one machine, but not the other. I'm thinking it may be because I upgraded this machine from Vista, but that is a total guess.