Will not treat a severe spider mite infestation

We have on our balcony a small corkscrew willow with strong spider mite infestation.
I first directed me to the leaflet and all leaves of the tree sprayed thoroughly with the water-oil solution. However, it was observed no effect that would be superior to the use of water-soap solution and could justify the price of the agent.

The spider mite infestation is so briefly back (probably more due to the humidity, as the NEEM), and then fully after a 3-4 days break again.
The short-duration population decline is also more likely to be connected to the previous wash the plant ...

Since the spraying did not seem to help, I have also added the irrigation water at the second application with Neem .... even so were the spider mites not to impress.

tl: dr
The product will not work for severe spider mite infestation ...
(And probably in low not ...)