I'm not sure I totally understand what you are after, so please clarify if the approach I suggest doesn't provide a solution.
The WHERE clause of your query restricts the dates between the date entered and 14 days from the date entered. This will not guarantee that you will get the next 14 dates. You might get less or you might get more.
What I would do is to find all of the dates greater than or equal to the date entered and then use another query to get the 14.
First, modify your current query as follows. Please note that I changed the Isdate alias to myDate. Isdate() is a function in Access, so you should not use the same name; it may cause problems.
Code:
SELECT Results.Result, Test.TestDate AS myDate
FROM Results INNER JOIN Test ON Results.ResultNumber = Sample.ResultNumber
WHERE (Results.Result Like "*A*") AND (Sample.TestDate >=[Enter Date])
ORDER by Testdate desc
Then create this query based on the query above
Code:
SELECT TOP 14 Results.Result, Mydate
FROM nameofqueryabove