Shared Workbooks

Hello I created a spreadsheet in Excel which up to 100+ people access and update

I made it shared so everyone can access it, but every now and again a user gets locked in the spreadsheet, sometimes even when they've exited this and blocks anybody from going into the spreadsheet unless it is read-only. The user sometimes doesn't get free from the spreadsheet for hours.

Why does this happen is it because Excel cannot cope with too many users going in and out and making changes or is it a problem with the server?