The problem here is not biblical..
On one level is spiritual.... because Satan has blinded them.
On a human level it's conceptual... because JWs simply cannot or will not grasp the simplest element of the Trinity.
Let me put it simply........
To a JW the "term" God must mean one person in order for God to be singular in any sense. That is how they see God.
If you try to suggest that ONE God could be THREE persons the concept simply hits a brick wall that stops them considering the concept or the possibility.......... and yet they actually live inside this concept and it's one that God devised!
The concept is simple.... A SINGULAR entity being comprised of MULTIPLE parts. Thus a drop of water is SINGULAR but it's made up MORE THAN ONE part........ as is true across the entirety of our universe. .... but more than that, this concept also applies to many human entities.......... A choir, a team, a couple, a congregation, an audience, etc. .. and the best example .... A FAMILY.
JWs seem perfectly capable of accepting that they have "A FAMILY" that is family SINGULAR whilst said family is comprised of multiple people. Consider A FAMILY of Father, Mother and Son. ONE Family THREE persons. JWs don't go around asking are there three families here? or Is there just one person? They know that is ridiculous..... yet when you explain that the concept of the Trinity is like this, they simply cannot comprehend it.........most assuredly because they don't want to believe it is or could be true. How do we know this is their problem? ... that's easy... because they ask questions like "Did Jesus send Himself?" or "When God said this is my Son..." was He speaking to Himself? etc.
The irony here is that "IF" JWs simply grasped the concept of the Trinity it doesn't mean they have to accept it's true, although that's why they seem to cling to their nonsense...... but if they got the concept they would at least stop asking the dumb questions.
On a last point, many JWs like to use a particular verse to claim Jesus cannot be God.. "No man hath seen God at any time, the only begotten Son"... The simple thought applied to this being... If nobody has seen God and lots of people saw Jesus, then Jesus cannot be God...............To JWs this is proof.
...but to accept this simple thinking you have to do a mental dance to avoid .."It is because I saw God face to face, and yet my life was spared" Genesis 32:30... The dance JWs perform on this verse is "well it wasn't really God, Jacob just thought it was God.. it was just an angel of God".... The bible does not say or even infer that Jacob thought it was God... but more than that who in those times, any times since or even these times has ever marvelled ... "I've seen an angel and lived"???? NOBODY.
.. but more than that.... if God CHOSE to include this verse and Jacob really was mistaken in thinking he had seen God and lived when infact he had only seen an angel, a whole bag of worms ensues............. Why did God choose to record Jacob being fooled and honouring an angel in such a way? Why did God not see to it that the right view of the events were recorded? Why did an angel change Jacob's name to reflect an experience with God when it was only an experience with an angel? etc. It's highly amusing yet profoundly sad to see JWs perform this dance for them to cling to the simple words of "no man has seen God".