I have bricked quite a few 16F627A's before I realized what I was doing wrong. I could program them once, then I could not write or clear them after that. Had to throw them away. It turned out that I was enabling timer 1 external oscillator which was disabling MCLR. I had no way to reset the chip with the programmer I was using, so I couldn't re-program them. Carefully review all your configs for the MCLR pin.