This is how I solved the same kind of problem I had on MS Access 2003 connected to SQL Server.
A similar problem was seen when I was using those tools. Just like you, I was able to add records from interfaces on MS Access but was not able to edit them due to a record lock. I kept getting a dialog message with the button for addition of data disabled. It also happened at manipulation of records from tables and I could not even delete the record that I had just entered from there. I doubted that some configuration on SQLServer might be doing this but could not really figure it out until I took another approach to fix it.
You know, MS Access 2000 or above has a useful feature called "Upsizing Wizard" that enables you to toss your Access tables into MS SQLServer. I guess that it does some extra works needed for clean server-client use of records. After I linked those tables onto Access thru ODBC, the negative behavior that I was struggling with was gone. (It's been good so far.) You can of course create link tables manually but you might have to adjust configurations on SQLServer and that might be something that is bugging you. Like I said, one way to fix it can be to use Access 2000 or above and create links with "Upsizing Wizard".