Time slot fee didn’t work

Hi, I am using this plugin. It’s useful. However, there is a minor problem.

Recently, there was a order placed in my website. The customer selected a paid time slot. When I check the invoice of the order, there is no time slot fee listed on the item list. Therefore, ther customer didn’t pay for the fee.

Therefore, I wanna know what is happening? Is there any solution to prevent this situation in the future.

In additon, my plugin version is 1.3.77.

Thank you very much!