How can I get WP updates from dashboard?

I can’t update WP or add or update plugins from the dashboard on a couple of sites I’ve set up — I can only update by downloading updates and uploading the folder via ftp. That’s getting a bit annoying with all the recent updates.

If I try to update from the dashboard, I only see “Unpacking the update…” after entering the log/pass information, but the process must time-out without notice or error message.

Are there changes I can make, or do I have to speak with the server Admin? if the problem is server settings, what do I ask for? One server is probably Apache on Apple. The other is Linux or Win., with a Plesk Panel.

Thanks