"A" button doesnt work can this be fixed?

Sure, use the following guide to get to the buttons rubber pieces and buttons.

Nintendo DS Lite Buttons Replacement

Make sure your buttons and rubber are clean and not sticky. If they are sticky clean them with rubbing alcohol and a paper towel. Make sure the hard black dots on the rubber are all even and not popped out. Then clean the brass colored "contacts" on the motherboard where the rubber makes contact with the motherboard with rubbing alcohol and a brush. You can see them on the pictures in step 16 here:

Nintendo DS Lite Buttons Replacement. Wipe the motherboard with a paper towel to ensure that any residue is removed. When the motherboard is dry assemble the DS Lite back together. It should work now.