Laptop fails to suspend

[Sam Smith]

I have an older Lenovo T520 laptop that I've ran Debian on for years and 
I have never had any issues with putting it to "sleep" or suspending 
when closing the lid. However after upgrading from stretch to Buster, 
suspend fails to work.

I run KDE and I've tried to suspend from the desktop using the built in 
"suspend" button (right next to "shutdown" or "reboot" buttons), as well 
as using systemd or pm-suspend from the command line but they all result 
in the same issue. The issue is that once the machine starts to go into 
suspend mode, the power led flashes and the screen then goes blank. 
Normally at this point the power led would stop flashing and would stay 
off, however it keeps flashing. I've left it for a couple of minutes but 
it never stops. Keyboard is unresponsive and I have to force it to power 
off (hold down power button). How might I go about trouble shooting this?

Thanks,
Sam