Maybe it's all the time traveling you've been doing! ;D Not really.
Honestly, I can't figure out why you can't order anything. Also, I've had the same problem with trying to send you letters.
EDIT: It's Saturday, which meands that K.K. Slider is performing tonight. Which song will you request this week? If you need a suggestion, try requesting "Forest Life" (without quotes). It's actually a song from the original Animal Crossing.