the problem i had was i had to push the button quite hard and only from the top. with the back off i noticed when you push the button it pushed the bottle which then pushed the battery a little. i tried it with the back off and it worked great. the back would not let the battery move.
i noticed the back had two little nubs thad kept the battery bottom from moving. i thought, hmmm, dremel time. i removed the nubs , with a little slippage, in about a minute.
it now works great with less than half the pressure and any finger placement.
i'll post those pics separately because i don't know how to put them between paragraphs.