[Yaffs] yaffs chunk error

hi guys:
I use lastest yaffs filesystem,kernel is 2.6.25. my nand flash controller driver is Freescale Enhanced Local Bus Controller NAND driver.and when i copy(or ls) files(more than 1MB?),the console appears:**>> yaffs chunk 62720 was not erased **>> yaffs chunk 62784 was not erased **>> yaffs chunk 62848 was not erased **>> yaffs chunk 62912 was not erased **>> yaffs write required 321 attemptsso why this happens continually ?