you can only see dna by the intercalation of a fluorescent dye like etbr. unfortunately the fluorescence does not last forever. So if you irradiate with UV light eventually the emitted light will fade so probably either ..you are leaving the gel on the light box for too long while you check it. Or the filter on the light box is aging and you are irradiating your gel with hard ( short wavelength ) Uv and quenching the sample.

One other possibility is re using old heavily used buffer to run checker gels and if bugs are growing in the buffer they might produce nucleases which are cutting your product. So Use fresh buffer and view gels quickly. Possibly post stain by sitting the gel in etbr buffer for a few minutes might improve the faded signal does occasionally work

try to run the gel under low volt i.e. below 150amp. sometimes overheating may also cause this problem. best of luck.

Thanx a great deal would keep you updated.