First verify that CLKOUT is correct connected to the buffer and the output from the buffer does not exceed 3.3v and is correctly connected to XCLK then verify that with an osciloscope (you should see a square wave) after that see if SIO_C and SIO_D work it may just be a quick burst but that is normal. Then see if you get a most square wave from PCLK there should be a very brief burst where PCLK stops every so often. This is because by default the ov7670 outputs unwanted dummy pixels that serve no good we don't want those dummy pixels. If PCLK never stops it may not be communicating properly but the part where it stops may be too fast to see I can not remember off the top of my head.