Are you seeing messages in red that say undelivered?
Unfortunately, the reason that you're seeing a "Not Delivered" status is that Group Broadcasting is still frowned upon by cellphone providers. These providers are concerned about their customers receiving SPAM (Verizon, AT&T, Sprint, T-Mobile, etc.), and while we've built our system to try and achieve a higher delivery rate (the limits on how many you can send per day, the group size, etc.), none of these are a guaranteed solution. From time to time, with regular use, it's normal to experience these kinds of issues.
When you see the "Not Delivered" status, it is because the provider has temporarily blocked your number. These will usually lift after anywhere between 15 minutes and several hours. In the future, when met with a "Not Delivered" message, a great first step is to wait 15 minutes, and attempt to send this message again. You can also flag these messages, which will move them to your flagged list and allow you to keep track of which Undelivered Messages still require your attention after waiting for the block on your number to be lifted.
Here are a few more reasons you may be blocked by cellphone providers:
-Using identical messaging in multiple broadcasts (when needing to send to more than 100 contacts, try to change up the messaging a bit to show the carriers that you are not sending out SPAM)
-Use of an email address
-Use of a phone number or too many numbers/dates
-Use of all caps (ASAP, HOW ARE YOU, THANKS, etc.)
-Use of special characters ($$, /, @, %, &, etc.)
-Use of excessive punctuation (..., !!, ???, etc.)