When do babies understands instructions ?

Hi mummies, i need some advice. My baby is 11 months and its quite difficult to get her attention. Sometimes we need to call out his name multiple times before he turn to look at us. We tried to ask him to pick up his ball, teach him how to stack boxes, teach him how to say byebye, repeatedly show him which one is cat in the book but he did none of the above. He can only mumble 'tatata'. Sometimes i was so frustrated when teacthing him and he just dont seem to understand. To add on, my PIL always compare him with his cousin who is 2months younger than him but are more quick in achieving his milestones. Are all this normal for baby at his age ? I really pity my son ☹️