It sounds like your data context is stale (it's retrieving the data from memory and not going to the DB).
See this question or this one for a possible solution.
Edit: Looking at your sample code, I'm going to guess that this has been declared as a page (or class) member. You should wrap this in a using statement so the object context can be disposed:
e.g.
using (var db = new dbEntities())
{
//perform work
}