Shared plan - saying no credit

I've got the $49.95 unlimited call and SMS plan with 20+GB of carryover data. I share this with a family member and in the past if they use anything that isn't included in this plan, the cost is simply added to the bill. However, in the last month or two, there have been funny things going on like receiving a message that said share is now off - but it carried on working. I am now seeing a message showing no credit, 251MB data etc. Where is this info coming from?

I've been fortunate not to have any major issues but the last couple of months of 2degrees issues have made me lose confidence big time. There are a LOT of niggly issues that need to be ironed out.

What's going on?