You finally found a great AE on your account-- it only took five months to get someone up to speed on the account this time. Then, all of a sudden, there is an account change. And that was the third account person you've had in 18 months.
Guess what! You're the training account. Or, you could be a difficult account to work for, or you don't treat the agency with as much respect as you should. Often the AE is moving to go to another branch of the agency to work on another (translation: more important than you) account. But nobody will ever tell you that.
AE stability is crucial. The AE is your conduit to good work throughout your agency. When you find a very good one, communicate to the agency that if he or she is moved off of your account, you may move the account off their agency. If the agency wishes to give that person a promotion, fine, but do not let them move a junior person in under your preferred AE and try and convince you that your person will still be your main contact. In three months, you will find yourself dealing almost exclusively with the junior person.
If you find that you are rotating through account people you may be the training account, but more likely you are not a right fit for the agency. As a client your expectations are different from the agency's process for meeting your objectives. Again, they will never tell you that, but like Animal Farm, some clients are more equal then others.
If you work for a company that has many divisions, and you are just not an important one, you get screwed. Often, large companies consolidate accounts at one agency. When they do this throughout a company with multiple divisions, the smaller, less important divisions get the short end of that arrangement.
Find ways to extricate yourself from such arrangements. If you are "not allowed" to use other agencies, ignore it.
You have a choice of working in a relationship that is painful, on which you are consistently training people, and not accomplishing what you need to do, or (ahem) for lack of a better phrase "getting some balls," enjoying working with your agency, getting more attention, and producing better work. I choose the latter. If you can't make the same choice, then you shouldn't be working in this business.
If you ever want to improve your position, it is always better to ask for forgiveness than permission.