How does neem work?
It's a thick oil from the Neem tree in India. If you can look for Einstein Oil, that's considered one of the better brands of Neem.

The reason i say spray the underside of the leaves is because that's where most pests and spider mites in particular will reside. On the underside of the leaf. Spraying vigorously once or twice will not clog the pores in the leaf. This usually happens if you get the Neem to water ratio wrong or add to much soap as a wetting agent.

If you spray only the top side of the leaves and never the underside than you won't get rid of your infestation.

Neem works by coating the pest eggs and preventing them from hatching.. which is why you apply every 2 or 3 days as this is the life cycle of well.. spider mites specifically.