Dr tried placing epidural during labour four times but it never worked. it made my right leg numb once and left leg another time. why didn't it work?

It didn't go : Into the right place. Sorry. Sometimes they can be tricky.

Your unique anatomy: An epidural placed for labor and delivery is inserted using our knowledge of anatomy. But just as we all look different, inside anatomy is also slightly different. The location of the nerves, epidural membrane, and bones in your back may be just different enough to prevent the local anesthetic from being injected into the right spot and then spreading to the nerves that must be numbed.

More: Also, an epidural is placed essentially blindly using tactile feedback as a needle passes through different tissue densities. For non-pregnant patients sometimes x-ray in the form of fluroscopy is used to aid in the success of the procedure.

It was not placed: In the proper space. That is the epidural space. Also may have been placed in the epidural space but the catheter may have migrated to a lateral position which explains why you had numbness on one side then the other. Some people are difficult for placement and/or the person placing the epidural may not have been skilled. Hard to say. If you have another delivery hope this will not discourage u.