Version 6.4 Message+ introduced a new type of communication feature (Chat or Enhanced Group) - which goes over the internet.
For people familiar with iMessage on iOS/iPhone, "Chat" is analogous to the "Blue Bubble" and the traditional SMS is like the "Green Bubble".
Now let me explain the scenarios in which some users will see problems after upgrading to Message+ 6.4.
Let's start with simple SMS - direct one to one text conversation with another user (say User B) :
There are other scenarios where "Chat" messages will get delayed or not sent out right away. These surface when either you or user B does not have either good internet connection and/or have mobile data turned off. Message+ 6.4.4 also fixes most of these scenarios.
One other scenario to check for if User B says that they are not getting messages from you, is whether they have paired another secondary device to also receive messages on - eg. a Tablet or an iPad with Message+. If they do, then they will need to have Message+ as their default messaging app on their Android phone to reliably receive your texts. We are working on removing that restriction in subsequent release of Message+.