On serve, the problem is they didn't develope in early age because they were too young. Once bad technique developed, it is extremely hard to change on pro level. Venus is a prime example, Elena is another. I don't believe she isn't practicing her serve. Serena's serve is just natural. JHH used to have weak second serve, but her technique was sound thus easier for her to improve.
As of coming to the net, don't just belittle the women, the men on ATP don't come to the net either. They don't develope when they are young. When they are older, at higher level Volley is difficult because its technique is completely different from any other stroke. Plus, they just don't have the instinct at the net.
So, coach should focus a little more on these two aspects of tennis when the players are still young.