other reasons:

1. overfilling wells

2. gel concentration

3. high voltage than required

4. adjust your MgCl2 concentration in OCR reaction

5. wells are not properly prepared.(in this case all wells should show smearing)

improper template concentration for pcr is the most common cause..thts my guess here because few wells r clean n few r not

Looks like DNA degradation, although I hope you have solved this problem already at this point :)