the gear that makes the robot rotate is below the gear box. That shaft that you see lift up when the robot stops walking has a gear on the end of it. when its down that gear connects with the walking gear drive when its up it connects with a gear that makes the body rotate. so either the gear is worn at the top edge so that when it lifts up it can not engage the rotation gear or the rotation gear is worn "bad teath" at the point that they mesh. You need to take a peak at what is going on below the main motor gear box to see what the problem is.
Grabbed this info before the auction disappears. sold for $1795.00 Robot Prototype Set
Vintage Japan Prototype Robot Set. All 3 Robots were found in Japan at the former offices of the "Junior Toys" Company. Each Robot appears in beautiful condition, complete, and includes original box. The "Junior Toys" company of Japan produced many toys during the 1970's. The first Robot in this particular series was the "Dynamic Fighter". The three Robots in this auction followed shortly after. It appears the darker blue version was one of the early designs which seems customized and has a sticker on the back which I had translated. It says the date, October 7, 1975 "Space Sergeant". Please email us with any questions prior to auction end.
The plastic nub piece is a press on that holds the body ON, if yours is cracked then thats why the body fall off. The robots body rotates all the time that it is powered ON if working right. there are a lot of videos of the robot in operation on youtube, here is just one.
So are we talking the 1980's? I have alot of the small plastic robots featured on some of the links you posted Joe but I don't know that they are Bruder toys. At least the Robotic Probe Pod has a name and manufacture now.