[raymo]

I am assuming that you are inserting the diagonal into the scope, and inserting the barlow into the diagonal, and then inserting the eyepiece into the Barlow.
You can insert the Barlow between the diagonal and the scope, and insert the eyepiece into the diagonal as normal. Most refractors will allow the use of a Barlow using one or other of these two arrangements, and sometimes both.
Note that the two positions of the Barlow in the optical train give different amounts of magnification. If I remember rightly one gives the expected 2x and the other gives about 1.5x.
There are also the so called shortie Barlows that can help with this problem, being much shorter than the usual Barlow.
raymo