It's being exclusively with Amazon no longer viable?

My sales have been falling over the past three months and I don't know what is causing this. As far as I know I'm doing the same things I've always done and the books I'm releasing are as well presented, covered and edited as any previous titles.
I've done a quick survey of my writer friends and so far everyone is saying there sales have gone up, not down, so this has to be something related to me.
The first thing I considered his that I need to rethink my marketing strategy. Occasionally I try to get a book into a BookBub  promotion, but they are always turned down. I think one of the reasons is that I only publish with Amazon exclusively and I'm not on any other platforms.

I've been told about D2D who will put my books up on every platform in return for a 15% of the net royalty. The good thing about this company is that you can remove your book at any time for any reason with no penalty. The first book in my new series, Ellen's War – Blue Skies and Tiger Moths, is going to go with them. This will probably impact on the royalties I earn from this book initially but if it works then I shall consider putting my longer historical books with D 2 D in future.
I am also going to organise a Thunderclap for this title as it did very well for my Victorian saga last year.
I find promoting my books very difficult – I come from the generation of women were brought up not to put themselves in the limelight. However, I have decided to be more proactive in future and just hope I don't overdo it.
I have taken out a promotion with Book Sends for A Suitable Bride and for Hannah's War. Both of these will run at the end of this month. I was turned down for the second time with E-Reader for a promotion with them for this book. They took the first one which was for a Victorian saga, I've no idea why they don't want to run one for my Regency series.
I post on my author page and on my regular Facebook page every day, I respond to other people's posts and also tweet a couple of times a day. I really don't know what else I can do as I don't think that blog tours work any more. I'm not sure if any of this expenditure and effort will make any difference. In fact, I think that the real problem lies with the fact that I am Amazon exclusively.
 I know another writer who produces seven or eight new Regency titles every year, isn't in Amazon select, and her sales are growing. In fact of the writers I spoke to none of them are in Kindle select so I'm coming to the reluctant conclusion that maybe that's where I'm going wrong. There are a finite number of readers subscribing to Amazon's library system and I think that all of them have now read  my books and there are not enough new subscribers joining to make the difference.
I think it's quite possible that being exclusively Amazon is no longer a good marketing strategy. Most of my writer friends are on many platforms; but I have avoided this as it's so easy to manage one's income and publication by doing it in one place.
What do you think? Are the days of making a fortune through being exclusively Amazon Published over? Doing free promotions was a huge thing four years ago and I was fortunate enough to get in at the beginning and I'm certain that is why I've done so well with my books. The first promotion I did I got twelve thousand downloads – if I do one now I'm lucky to get a thousand.
I would be interested to hear your views.
Fenella J Miller