Cannot finalize a purchase on MarketPress

Hi there,

People cannot buy items from my website. I am using MarketPress. I use PayPal Express Checkout.
Everything is goes well( add to cart, checkout, payment, etc), and I should get the order( and the money in my account)...BUT nothing happens. The item is still remain in the CART, and no order or money arriving, also it DOESN'T charge the buyer account either!
Please help me out with this issue.
Thank You