This might be a very silly question but I have to ask it anyway. I have 2 forms both with queries as the record source and both allowing edit, update, data entry etc but one of the forms has the new record button greyed out - Why ?
If the new record button to which you are referring is the one on the far right of the navigation buttons then it is greyed out because the form has its allowadditions set to no. If it is a button on the form that is visible in design view, then its Enabled property is set to no.
If the record source of the form is a query then open that query. Try adding a new record directly to the query by adding in a test line at the bottom. If there is no blank line at the bottom, then the query is not updatable. Usually this because of a sum or a sub query that has a sum or other total function. Post the SQL text of the query if you cannot add a record to the query and you are still having trouble.