To understand why your money isn’t in your account, you have to understand how the money actually gets to your account to begin with. It’s not an instantaneous thing, despite living in a primarily digital world, it’s a process. The money has to go to a clearing house, and then to the writer’s bank, and then finally the funds are transferred to your bank. Since the process takes time, the money only shows up a few days later. And sometimes, there’s a lag at one of the stages, taking longer to get to your account.
Other reasons a check may take longer to clear is if there’s a hold on your account, the check is from out of state, or for some other reason including bank clerical errors. If you don’t see the money in your account after standard business processing time, be in touch with your bank to ensure there wasn’t a mistake on their end.