Set Priority for constraints. By default all constraints have 1000 points of priority. It's call conflicts. Just set priority for constraint V:[UIButton]-(10)-[UIButton]
like height(750 points) and its solve your trouble. Or set not equal(==) constrains, but set >=,<=.
p.s. in visual format constraints between buttons needs like @"V:[button1]-(10@750)-[button2]"
or @"V:[button1]-(>=10)-[button2]"