Why is Tesla asking for payment for SC'ing when I have free SC miles left?

[crouchy]

I had 2000 miles, I was under the impression that if you got a referral it extended them by 6months, but they both expired at the 6 month point

 

[Tony Hoyle]

That's odd.. I had several and they all added together (due to expire next month, so ~22 months later).

 

[Drew57]

Any I received have definitely added 6 months each time but up to a finite limit. A collection this month has added miles but not extended the date, previous ones have done both.