First of all, I'm aware that there's already very similar questions out there, but I've tried the answers and they don't seem to work. My keyboard doesn't have PgUp/PgDn keys, and for some reason scrolling doesn't do anything. I've also tried adjusting it manually using the thing at the bottom of the tool panel, but that doesn't do anything either. Any suggestions?
If you have trouble using the standard keyboard shortcuts you can adjust your settings to suit your keyboard.
In File->User Preferences under the Input tab you will find a group called Transform Modal Map, under which you will find the shortcut settings to adjust proportional influence. You can adjust these to anything that works for you, be sure to Save User Settings so that the changes are available every time you start blender.
It's very strange that it doesn't work for you (as @pezzatron said, be sure, that your proportional editing circle radius size isn't too big for you to see it). If you can't do it using the MouseScroll or Page Up/Page Down buttons you can still do it using the Proportional Size slider via Tool Shelf's (T) Translate panel.