Problems with Sendgrid's Service

For the last few months, I have been having problems with Sendgrid's email delivery. Worse, I have contacted support over and over and never get a clear answer. When I raise specific problems, they regularly give me non-answers like "Well, we deliver x percent of emails", ignoring the emails with problems. The specific issue has to do with using Sendgrid for newsletters. I have had dozens of my customers say they no longer get my emails, despite the fact they always got them with ConstantContact, a provider that I would have thought would be less strong than Sendgrid. Two days ago, I asked to speak to a Sendgrid manager but still no response. It's a combination of technical issues and poor service. Can anyone help? Anyone with similar experiences or recommendations?