What could be preventing updates and inserts to a table without anything visibly blocking the process?
I will prefer to track blocking with any of 2 below rather than SP_who2 or Activity monitor:


Use Sp_whoisactive which is a procedure written by Adam Machanic. Once created , you can gather the data to analyse the blocking using:


a. Create one destination table

b.Create Your Loop to Periodically Log Data

c.Set Up Your Query to Look at the Results

On how to get these details captured, read here.


You can use SQL Server Blocked Process Report Viewer written by Michael.J swart which apart from finding help you in analyzing the blocking root caused in very helpful manner.


You can read the article Okay, You’ve Found Blocking, Now What? to get more detailed info.

Hope these 2 will help as they have been doing us for quite long.