How can I correct my juggling?

While practicing juggling I discovered two things:

• I am currently horrible at juggling
• The ball seems to have a backspin when I hit it

The two seem related.

Let me explain:

When juggling, the ball seems to hit a place on my foot right below my laces. I try to keep my foot firm and myself in a stationary position — yet the ball spins off of it and back towards me – usually bouncing off my shins. Maybe my foot or toes are curling upwards?

What is the root of the problem here and what would be the most effective way to combat it?

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I have looked at this question, and it was helpful, but I do not think mine is a duplicate, as that question asks what the best way to do this skill is, while I am asking about addressing a particular problem I have encountered.