It's possible that it is an adjustment but usaully if it's shift linkage and you are locking it in 2 gears at once it wont move at all and create a nasty reaction. The shifter has a hole that you can put a drill bit or equivalent through and adjust the shift rods from there. Easiest way is to put the shifter in neutral, disconnect the shift rods at the shifter, make sure the shifter levels are lined up and slide the bit in the hole (it is at the top of the levers at the base of the shifter housing-visible in the attached photo). Then make sure that the trans is in neutral and adjust the rods where they slide freely in and out of the appropriate shifter level. Once everything feels good, remove the bit and shift it through the gears to make sure it feels good, then try it running without too many Rs just in case. If this doesn't do it get the builder to look at it in the car. If it's their problem they should round trip it for you.