Declined challenges

Question

Most of the time, opponents are declining the challenges. Or challenges are getting expired.
How to choose active opponents?

Answer 1

try choosing someone with a level close to yours
when you win against a much higher level you might gain a lot of XP with the risk of only losing few points if you lose.
while the higher level opponent will lose much more XP than you, but if he wins, his/her point gain will most likely be very low.
TL;DR
when the level gap is big:
low risk-high reward for low levels
high risk-low reward for high levels