Given the age of the P2417H, that stand spring has loosened over time. There is no adjustment of it.

While you may not be able to increase the friction in the stand, you can clip a small office wired backet on the back of the monitor to hold stapler, scissor, enough to increase the monitor weight.  That should allow you to keep the monitor sets at the level of your desire.

It does not have to be a wired basket, you can use anything that will work in the same manner with your creative mind.

Yes, adding weight is a good idea. 

You could also purchase a replacement stand off ebay.  Take note of which model number is selected on the square mounting point, as the movement will be adjusted to that model. 

Before you throw out the old stand, you can remove the plastic strip in the middle of the 2 arms.  There is a screw visible in the mech which changes friction.  I think you move screw anti-clockwise

That thing happened to my monitor stand too. you should try using weight on it or tying it from the bottom with some thread in the start the spring is much hard so it does not stay at tthe same place as we set it but it adjusts after a few time of usage.