When using the Yammer Get All Messages Action I only get 20 messages for some reason (we have way more).
I've tried changing the Page parameter but the same information is returned.
I have also tested on my side, and as you have described, Get All message can only get a maximum of 20 messages.
The reason is that when sending a request to Yammer, only can be 20 messages returned.
I am afraid that there is currently no way to achieve your needs through Flow.
In addition, please check this link and see if it helps:
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