I have the following C# code (from a library I\'m using) that tries to find a certificate comparing the thumbprint. Notice that in the following code both mycert.Thumb
Sometimes when we insert data in database it stores some spaces like "question ". And when you will try to compare it with "question" it returns false. So my suggestion is: please check the value in database or use Trim() method.
In your case, please try: mycert.Thumbprint != null && mycert.Thumbprint.trim().equals(certificateThumbprint.trim())
I think it will return true if any record will exist.