I have several of them and once in a while I find a similar instance to what you mention. Couple of things which I might have missed above,
1) I have found the biggest issue to holding the blocks apart to be burnt lube on the pins or in the holes the pins should go into. I used a q-tip with a tiny bit of 0000 steel wool on the end to polish both the holes out nice and shiny. I also used a bit to polish up the pins. When I preheated the mold I used only enough lube on a q-tip to see the pin and hole change color and all was fine.
2) Look the holes and pins over closely for any peening which may have occurred. Sometimes you get into a good rhythm, and it is easy to shot the mold up several times with just a bit of misalignment. As this happens the pin will hit the edge of the hole resulting in the slightest little bur being raised. The problem is once it starts it will get a little bigger as time goes on. As the mold heats up the pin and the hole will grow in size with the pin getting larger in diameter and the hole getting smaller. Similar to the above cleaning, you can use a small piece of emory cloth to polish both the pin and the hole to remove any sign of a bur. You want to use something in the 400 or so grit and go slow. I rolled it into a tight little tube to polish out the holes and simply wrapped it around the pin. it only takes a couple of seconds, so don't get overly rambunctious.
All that said, the most recommended lube I have seen for molds is Bull Shop's Sprue Plate Lube. He is a member on Castboolits, and may or may not be back into sales. There are quite a few who also use a high grade synthetic 2-cycle oil as well. There is also another member on CB's who goes by RandyRat who is selling a good one. Thing is to simply experiment and find something that works. I have used LLA and other types but found they will get gummy and sticky over only a few sessions. The others I have not found this to be an issue.
Hope this helps.
Mike / TX