Virtually all authors (and actors and musicians and artists) moan about their agents occasionally, and some have very good reason indeed to moan.
But all agents have known authors who are never satisfied, who don't understand what an agent can and can't do about your career and your relationship, and blame an agent for everything bad that happens, regardless of whose fault it is.
(Mind you, agents in some ways have only themselves to blame, because they are notoriously dreadful at confronting their authors with bad stuff, from rejections to brutal truths about what they see as the writer's long-term prospects. So the writer doesn't actually know what's going on, only that nothing much seems to, and feels the agent's incompetent at business, when in fact they're chiefly incompetent at communicating.)
So there's always a risk that you come across as the latter to any prospective agent (and it's a small trade), when you're actually the former, and justifiably fed up with what's been going on.
So I'd suggest that it's important to make sure you can say with reasonably honesty to any prospective agent who you're approaching that you and Agent A are parting amicabably.
If it were me, (and it isn't) I'd try to have a state-of-the-nation conversation with Current Agent about the present and the future. At the very least you'll find out what actually happened in these various events, which will be stuff that a new agent will want to know so as to get a handle on your situation thus far.
And the chances are that you'll both agree that you've reached the end of the road, and you can then go off looking for someone else.
The wrong way is to say nothing to your current agent, and go off to lots of other agents telling tales of his/her incompetence, laziness and failure having betrayed your sure-fire bestseller status...
Good luck!
Emma