DIAGNOSIS The metal button latch on the handle that locks the wand is either stuck, broken, or not engaging with the wand’s notch.
FIX Press the metal button on the handle repeatedly. Spray a small amount of electronic contact cleaner or isopropyl alcohol into the mechanism while working the button. This frees up dirt and dust. Let it dry.
IF THAT DIDN'T WORK Inspect the notch on the wand itself. It may be worn down or damaged. Also, the spring behind the metal button in the handle can break. This requires disassembling the handle to replace.
HEADS UP Never use a lubricant like WD-40 on the button. It will attract more dust and gum up the mechanism worse. Use a cleaner only.