This seems to be common with the Lyman GPR set triggers. It is my only complaint with mine.
On mine, right behind the rear trigger there is a set screw in the trigger plate.
By screwing this in you will reduce the amount of travel the heavy spring will have and it will reduce the amount of force needed to set the rear trigger a little.
On mine screwing it in helped but it was still a bear to set so I used my Dremel with a fine grinding wheel to reduce the springs thickness.
If you do this, you will have to remove the trigger assembly.
If you decide to remove the trigger assembly you only have to remove the trigger guard and the one wood screw at the rear of the trigger plate.
You do not have to remove the tang screws.
When thinning the large spring be careful not to thin it too much. A little makes a lot of difference.
Also be sure the grinding marks are parallel with the long axis of the spring.
The little screw between the two triggers controls the amount of engagement the front triggers catch has with the rear triggers latch.
It does not change the pull weight of the front trigger.
If it is screwed in too far the front trigger won't be able to catch the latch on the rear trigger so 'setting' the rear trigger will be impossible. It is safest to allow the front trigger to get a good bite on the rear trigger so the gun won't accidentally fire from some small bump.
My Lyman GOR trigger also has a feature not found on most set triggers.
The front trigger has a large "hook" that extends back behind the rear triggers 'blade'.
This hook is intended to keep the rear trigger from moving far enough to strike the locks sear and release the lock if the front trigger is not pulled.
While the idea is good, if the front trigger adjustment screw (between the triggers) is adjusted so that almost no force is needed on the front trigger to release the lock, this hook may prevent the set trigger from firing the gun.
This is another good reason not to set the little screw between the triggers too far in.
If you don't like the Lyman setup, you can replace the trigger assembly with one made by Davis.
They are superb triggers that are easy to set and those who have installed them say they work flawlessly. They will set you back $40 or so.