Contacts must be opted in to receive broadcast text messages through ActBlue Field Tools. This means that each contact must possess the built-in Field Tools tag called "Opt In (Broadcast)" in order to receive a broadcast message.
The easiest way to ensure that all contacts on a list are opted in is to check the "Is this a list of opted-in subscribers for Broadcast Texting?" checkbox when importing a CSV contact list - see here:
If that checkbox is checked, every contact on the CSV list being imported will have the "Opt In (Broadcast)" tag applied automatically by Field Tools, and they will be opted in to receive broadcast messages.
If you forgot to check this box when uploading a CSV contact list, no worries! You can simply re-upload the list, check the box, and all of those contacts will receive the tag and be opted in. Remember that Field Tools merely updates existing contacts if they are re-uploaded (more on this here!); so after re-uploading, the contacts will be opted in on any and all lists that they are part of.