Network Shared Folders-can copy to/from one but only from another

I have two computers: Upstairs and DOWNSTAIRS. Due to previous
recent advice on Tech Republic, I was able to copy to and from each
computer’s SharedDocs folder. Thanks. I then had to rebuild
Upstairs–it was long overdue.

Upstairs is XP Home and DOWNSTAIRS is XP PRofessional, both SP3.
When I got UPSTAIRS rebuilt, I found:

1. While sitting at Upstairs, I can copy to and from the DOWNSTAIRS
shared folder.
2. While sitting at DOWNSTAIRS, I can copy from the Upstairs shared
folder but cannot put anything into it.

How can I arrange it so I can put things into the Upstairs Shared
Folder from the Downstairs shared folder?

Please let me know how to work this one out.

Thanks