Hi,
the magnets have a north and a south pole, so it is important to assemble these in the right way. May be in old blocks (wrong marking / assembly or by user manipulation?) a few of them may fail. Old magnets had a square embossed marking (= inner side), newer ones a vertical black line (=outer side). While assembling it is a good practice to test the right condition with a second (good) block.
Best,
G