Phone calls on my iwatch not successful. the caller can hear me fine but I can't hear caller well on my watch.

When I receive a phone call to my apple phone, my iwatch rings and shows caller name. I can answer the call on my iwatch by pressing the green button. The caller can hear my voice fine as if I was talking on my iphone but I can barely hear the caller. So I cannot use my watch for phone calls. How do I turn up the volume for incoming voice on my iwatch?