Question is if the person building you a833 together did something wrong?

My :

Of course you should first make sure, linkage alignement is correct. If you're under the car and look at the linkage while someone puts reverse in (with engine off) you can see if the lever will move a good way (pointing backwards as it was pointing forwards before).

You can make sure if the lever at the tranny does not slip on the bolt that goes into the case. May be you will take the lever off for seeing if lever or bold are not "round". And of course make sure nut is tight.

As a last test before moving the transmission out of the car you can disconnect linkage and drive shaft (take care: a certain amount of oil will come out!) and move reverse lever by hand and feel with the other hand at the output shaft (moving forward and backwards) if reverse "goes in". May be you can feel it with drive shaft in place?

I recommend not to try to remove the side cover off the transmission while it is in the car - it would be very difficult to put it back with the forks in proper position.