Thank you for bringing this issue to our attention. It is possible you have some sort of block for unknown recipients set up.
It could also be possible that the specific short codes for those recipients need to be manually added. For example, if in the past you have texted "STOP" to any automated messages from those short codes, it would block them from being usable until you registered back for their automated services.
Please feel free to send us a Private Note, or reach out to us on Facebook or Twitter so that we can assist and take a look.