The latest, released today, is 6.4.3 - make sure yo are on the lastest version and see if things improve.
If not, go back to the stock app or choose another until Verizon messages+ gets back on track.
DO NOT GO BACK TO THE STOCK APP. I tried this and it completely prevented me from getting ANY texts. I'm furious with Verizon for doing this, I'm stuck in this disgusting limbo where I'm only getting a few texts from people and none at all for others, but can't even discontinue the app. I'm terrified to uninstall the updates (since you can no longer uninstall the app at all) and others have suggested deleting the account, but that's permanent and will guarantee cause more problems.