$$ extbf{Explanation:}$$ Following are the possible reasons: $$ ext{a)}$$ The gel plate may not be exposed to UV light. Ethidium bromide intercalates with DNA and further give flourescence only when exposed to UV light.  $$ ext{b)}$$ Electrodes may have been put in the wrong orientation; with anode or positive electrode towards the loading well. $$ ext{c)}$$ As DNA is negatively charged, It move towards the positive electrode. When the anode or positive electrode is near the loading well, separated DNA may move out of the gel.  $$ ext{d)}$$ The sample may be contaminated with protein or RNA. $$ ext{e)}$$  Improper loading of the sample in the wells.