Just a quick couple of tips for those of you with signature panels / write on strips, who seem to be having trouble actually writing on the strip. The following are reasons we know of:

Reason the pen may not work on the strip: Grease off fingers may cause a layer between the write on panel/signature panel and the pen – basically meaning than the ink can not stick to the strip.
How to resolve the issue: Try to handle the plastic cards with non greasy hands, you may need to wash your hands and try another card.

Reason the pen may not work on the strip: The pen you are using on the strips may have fibers on the end of it from use on laser/photocopied paper. Fiber build up can prevent the ink being released from the pen and/or the pen nib skidding across the signature panel.
How to resolve the issue: Wipe the nib of the pen with a clean cloth and try again

Reason the pen may not work on the strip: Perhaps its not a signature strip / write on panel at all! Maybe the customer never opted for one to be applied to their plastic cards – shame on you!
How to resolve the issue: Order plastic cards with strips in place next time!

Sorry I know that last one may not be very useful but just wanted to make sure you actually consider that this is a possibility!