I've had that problem with Samsung messages before the beta. I would just not get texts from some people, then move back to Google messages and all those texts I didn't get flooded in! Never figured out why that was.
Don't know if any of these fixes worked for you but for me, after messing with some settings I found that in the mobile network settings then in network operators setting turn off "select automatically" then pick your carrier. All my text flooded in in the Samsung messaging app.
