Why can't my Bittle walk well?

Hello.

I was able to complete the Bittle assembly. He can walk slowly. But he can't walk at normal speed. He loses his balance while walking.

I tried Calibration several times but it didn't work. I suspect this is due to the right hind leg. It looks to me that the right hind leg is behind the left hind leg.

Is there any other possible cause for this? please tell me.