I have a big (5’ x 4’) dry-erase board in my office. There’s about three years worth of scribbles on the board. The markings won’t erase.
How do I clean the board?
BTW: A few years ago I had a different office with a different board, but same situation. I tried cleaning it with a solvent and succeeded in ruining the board.
Expo makes a dry-erase cleaning solution. Based on its smell, I think its a very diluted alcohol.
Have you tried warm water?
Maybe you were using a Sharpie marker instead of a dry-erase marker…
Fingernail polish remover cleans those things right up.
Any office supply store would have what you need to clean it.
And here you thought they were useless anymore ! 
The stuff from office supply that they sell to clean them is only good if you clean them regularly with it. It works great on my monthly calander board when I wipe it off at the end of the month.
But the big board behind my desk with notes and phone messages and web site address and scribbles from my kids only gets wiped about two or three times a year and the only thing that gets it clean is fingernail polish remover.
The answer is right in your hand. I don’t know how much you have to clean, but the dry erase markers themselves will clean the board very nicely.
The old theory of clean dirt with dirt? Never heard of that? How about to clean oil off a driveway, use gasoline. Petrol product to clean petrol stain. Anyway, I digress…
Using a dry erase marker, write over the lines you want to erase, then immediately wipe it off with an eraser. This has always worked very well for me. (certainly better than the solvents they sell, IMHO)
E3
Rubbing the board with a pink pencil eraser will do it as well.
I was cleaning off a board at work using one until I discovered Enright3’s method was MUCH faster.