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That model scope package comes with a 3x barlow of hideously cheap design and build using plastic optics. a 2x barlow may work to focus better, if you're winding the focuser out and hitting limit before getting focus you need to get a small extension tube to bring the barlow out a bit further to the focal point. This is a bottom of the range scope , you are paying for Nat geo to put their logo on a generic unbranded product, so what it comes with is not necessarily what it is capable of using. Mostly likely someone somewhere said "chuck in a few extras, we need to shift more of these scopes". The scope tube should be ok, but everything else is made down to a budget only, tripod, eyepieces, finderscope, barlow, etc. I dont recommend buying a small eyepiece eg below 6mm, as you get blurring anyway from the atmosphere. A barlow magnifies this blurring anyway and people blame the blur on the wrong thing.
Yours certainly seems to be an issue of not reaching focus with the barlow. As Bojan suggests it could be a colimation issue which means the optical path is a tad longer or shorter than it should be meaning when you add the barlow you're now out of the focal point range. the build accuracy of cheap scopes is not great and if you've bought this second hand someone else may have messed with things without understanding. It does seem a distance problem wherby the focal point of the scope is not in the small range it was constructed to have, which can only really be done by poorly collimating the scope.
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