Well, that is interesting, I use Cinnamon as well and all the custom keyboard shortcuts I have created (11 of them) work perfectly
Strange things computers aren't they.
I don't know why this is, or even if it is significant or not, but if you search for 'keyboard' in the main menu there are two entries which look different to each other. When you open them both one is just titled 'Keyboard' the other 'Cinnamon Settings Keyboard'. I put all my shortcuts into the 'Keyboard' one not the 'Cinnamon' one, and they all work.
Have you tried that?
Fujitsu Lifebook AH532 Laptop. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Ubuntu12.10 (Unity), Mint14 (Cinnamon), Manjaro (Xfce).