Today when I sent some text messages they showed the checkmark and the letter D above it, I would presume that ment delivered? The ones I sent out today just had the checkmark and no D and I think all of thgem were sent succesfully! Is this a known problem?

The D (delivery notification) doesn't work for all carriers and/or devices.
Typically, the receiving end needs to be on the same carrier and also have a device that supports delivery recipts.

I have never had a problem with it before! And I text alot of diff people on other carriers and almost always get the D symbol after sending the text! I just sent 2 to the same person and one has the D and the other one dose not?