It seems this is happening to a ton of people and Verizon is doing nothing to correct this.
Like others, this started happening about the 1st Week of July for me.
I have a Samsung Galaxy 9 Note
I'm on Version 22.214.171.124 of Message+ on my Windows 10 laptop
It's a Dell with 500GB and 32GB RAM
(Both devices loaded with memory and no problems)
Same problem ... worked fine (mostly) on both devices for a year, and I have been using message+ for well over 2 years (including a different phone)
All of a sudden, it quit syncing ... period, and JUST LIKE everyone else here.
This is a VERIZON Problem, not a user problem.
Please ELEVATE and FIX the problem! I used this tool constantly for business when I am in meetings! My ATT friends have no such issues with theirs.