An Strange issu about nStatus

I used JTAG to download my FPGA, after that give me below information:"config_done can't go high". I checked the nConfig and nStatus signals, and found that nConfig was high and nStatus was low after power on. I have two devices on the JTAG chain, so i don't know which device doesn't release the nStauts pin. The most strange thing is that i can download these two deivces half day later, the config_done can go high sucessufully. But the nStatus is still low. The FPGAs can work well, it's very strange.