pull out spray tap keeps working loose

[BillyBuggins]

We have recently installed a pullout spray tap in the kitchen sink, and overall we are very pleased witrh the flexibilty it provides.
The only problem is that the retaining/securing nut works loose as the tap is swivelled left or right.
No matter how many times I tighten it, in a few days the tap works loose again and the whole unit becomes wobbly.
Either I have loaded the washers and nut in the wrong order, or there is something else.
The instructions say not to overtighten the large nut, presumably because it will distort the barrel's moving mechanism.The only other thing I have thought to do is place a piece of ply with a hole the dimension of the stem before the last washer and securing nut.
On the principal that the plywood would provide greater friction and prevent the nut from loosening...
A bit "Heath Robinson"ish I know, but what else do you suggest?

[PresleyT]

Would locktite or something similar work to keep the nut from loosening? You could ask someone at the hardware store what would work best because I know they make different types depending if it's around water or high heat or other conditions. You also might want to look at the instructions just to make sure the washers and nut and in the right order as that could be it.

[BillyBuggins]

thanks Presley,
I will go through the washers again to check.
Applying locktite might make future removal difficult.