SKS firing problem with Egyption ammo

I have a Yugo SKS 59/66A1 rifle. I have had great luck with this rifle the past 2 years. Yesterday I went to the range. I put 40 rounds thru it with no problems. The next ammo I shot with it was Egyptian 7.62X39 FMJ. First time I had ever used this crap. Five of the first 20 rounds all misfired. I have never had this problem with this rifle before. After one misfire I attempted to open the bolt to no avail. The bolt would not budge. I field stripped the bolt carrier and bolt in order to clear the firearm. The firing pin was no longer able to slide easily back and forth in the bolt. I reassembled the rifle and continued. Another round chambered. Once again a misfire. This time I was able to open the bolt but it was not easy. I won't be using Egyptian ammo ever again! I brought home the rifle and field stripped it. I soaked the bolt and the pin freed up ok. The rest of it appears ok. Any ideas as to why the bolt would'nt open up? The rifle is in excellent condition and the firing pin has always been clean and free to move. Perhaps an over tight cartridge case at battery causing extraction issues??.

9x19mm (Luger) ammo from Egypt was equally bad, at least in Glocks. Way hard primers!