key fob wont start

Please help, my key fob for Mercedes A class 1.5 is playing up, it unlocks but doesn’t lock, I don’t know if it’s key battery or button or something else? Please advise what should I do??

Hi guys, I have a 2002 W210 E320 CDI, my every day key fob works fine but got a bit of fright over the weekend when I couldn't find it and dug out the spare key only to find that it would only open the car but when put in the ignition there was no "zip" noise. TG I found my working key so...