I think it's likely that the inconsistency is just due to the fact that your T is busy at some times more than others. Texting is pretty immediate- they tend to get answered either right away or not at all. I know if I get a text and don't respond soon after, I usually forget about it later on. I only check my texts when I see a notification. It's probably the same for your T - it's just the nature of texting.