First thing is check in Applications folder for " Install Monterey " Application - If it exists.
If it does, Drag and Drop t Trash and Empty Trash.
There are factors:
1 - Like poor Wifi connection
2 - Slow wifi connection
3 - Too many other people downloading at the same time and Apple Severs are overloaded
4 - Software on the computer interfering with the download process including AntiVirus Software
5 - Running behind a VPN which may increase Packet losses and file corruption
6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer.
7 - Best Results are via Ethernet Cable to Router
8 - Empty Space Requirements >> If upgrading from macOS Sierra or later, your Mac needs 26GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44GB of available storage.
9 - Does the >> Computer Qualify for the Monterey upgrade