How do I stay published with lackluster sales?

I am a published author (4 books published by major houses) and have a poor sales record. My agent has shopped my last two manuscripts without any offers or real interest from editors. Is it me, or is it my manuscripts? When editors are pitched books by authors with lackluster sales, is there any hope? Or would it be advisable to pitch the book under a pseudonym, as if I were a debut author?

The first rule of Real World of Publishing is this:
Selling a book is easy; Staying published isn't easy at all.

I repeat this to disbelieving interns and agents every single year. After they've been in the trenches for a couple years they slink into my office and  hiss "yanno, I thought you were just an old curmudgeon, but you really were right about that." Then we have a drink. Or ten.

But I digress.

Since I know your work, and like it, it's not your books.
It's your sales record.

A new name is only going to help you once, if that.

You are now at an important junction in your professional writing life.
How hard are you willing to work to stay published?
If you only want to write books and not do a lot of marketing, you're going to take a different path than if you want to stay published by a large publisher. Think of it as stepping down to the minor leagues to keep playing baseball, or retiring from the majors.

To keep playing in the majors you're going to need to build your market.

If you resent this, and think it's not fair, I don't fault you. It isn't fair. But fair plays no part in success. If life were fair, every child would be above average and health care would be free. [And Colin Smith's eldest daughter would be my live-in pastry chef while she attended the conservatory.]

It's incumbent on you to SHOW editors that those lackluster sales are a thing of the past.
You build a mailing list. You build a robust public presence. You have articles in other outlets
that build your name recognition.

Lackluster sales plague 85% of the published authors. If you want to get out of that crowd and stay published, it's doable. It's just YOU have to do it.

Generally you'll start with a marketing plan plan (oops!).  Marketing is not something you need to devote an hour a day to for five years. 10-20 minutes a day will work just fine. The trick is consistency.

One of the most successful writers I've ever met is J.A.Jance. She worked her way to the top of her category, and right on to the NYT Bestseller List through her own indomitable efforts. I have the utmost admiration and respect for her. She was a relentless networker.

My very favorite JA Jance story takes place on one of her  book tours in Portland. I casually mentioned a new Barnes and Noble was opening at the Tanasbourne Mall in a week or so. Well! Nothing would do but that we trekked out there, knocked on the door, introduced her (well, me too) and inquired if they had her most current book. Well, yes they did, and sign she did, and thus when the store opened to the public, autographed copies of her book were on hand.

I saw the tour book she kept. It had the name of what seemed like every bookstore up and down the I-5 corridor, the name of the manager, the book buyer, the event planner. She knew EVERYONE. And those bookstore people knew her too.

Now, that kind of hands-on bookselling and touring doesn't take place much any more. The networking is largely on social media. BUT there's no substitute for personal contact, and if you've visited the 50 bookstores nearest your house, introduced yourself as a local author, picked an indie to be your home store (the one you direct people to for signed copies etc) you're well on your way to jumpstarting your career. No, it's not writing. It's not plotting. It's not making sure your characters are three dimensional. But it IS as necessary to a writing career as a terrific novel.

The only question now is, do you want to do this?
Or rather, I know you don't want to do this, cause hardly anyone does, but are you WILLING to do this to keep your career from ending?

It's a tough question that requires some real soul searching.
No one can answer it for you.
No one can guarantee the results.
And it's not failure to decide you want to spend your time and creative energy doing things a different way now.