Are you sure the problems a weak spring? Have you used other factory ammo in the weapon? Have you stripped the bolt and cleaned it thoroughly? I once had a guy with a rifle that was not firing. It was a new rifle of good make and the model gave no reasons for the light hits. I found the bolt handle touching the wood of the stock. This kept the rifle from going completely into battery and the interlock ate a large percentage of the strikers energy before striking the primers. A tiny bit of work with a chisel and the bolt closed completely and the rifles failures to fire,,stopped.. If indeed you checked it out and the rifle has the same problem when using factory ammo the the first order of business would be a call to CZ-USA. They have a website also. I've found them most helpful in my some what limited dealings with them. I bought a 527FS in 22 Hornet and it was superbly accurate. A second FS in 223 with the single set trigger convinced me the set trigger was a good thing. The Hornet lacking this I made a call and was told that all 527 rifle had the single set trigger when sold.. After a bit of discussion I was told they had indeed sold a few with the adjustable trigger. I decided to leave well enough alone..
