There is a fairly large gap between constantly asking somebody what's wrong and asking once because you've noticed that your friendship has changed.

I'd ask them.

Don't make the accusation ‘you're avoiding me’. Tell them what you've noticed. You've been speaking less, plans have repeatedly fallen through and you're wondering whether everything is all right between you.

There could be something happening in their life that has absolutely nothing to do with you. Equally, they may have pulled away for a reason they haven't discussed.

Either way, guessing won't give you an answer.

Ask once and give them an opportunity to be honest. If they say everything is fine but continue making no effort, I'd stop chasing. Friendship doesn't have to be perfectly equal every week, particularly when somebody is having a difficult time, but over the longer term it shouldn't depend entirely on one person keeping it alive.