Possibly the buffer contains too much salt and the current has dropped. Check the current on the power supply

• Prepare fresh buffer, don't reuse the buffer.
• Make sure the electrodes are covered by the buffer.
• Check that the gel is horizontal and there is no varying condition i.e. heat /cold in different portion of gel while polymerization, as this might vary the polymerization process.
• Presence of impurities, such as salts or proteins can affect the movement of the DNA
• Make sure agarose is completely dissolved, solution is clear and there are no visible particles.

In addition to Paul Rutland's suggestion, it could also be that the conductivity of your electrophoresis set-up is too high because of too much buffer on top of the gel, which would reduce the resistance. The power supply may then reduce the voltage applied to the gel if the set voltage implies a current exceeding the maximum value of the equipmemnt

Thanks Mr Paul and Mr Pierre. I checked the current it is 42 -46mA (which is comparatively lesser than other machines). But the thing is, in my machine I can only control the voltage and current is automatically adjusted. I am dipping the gel in buffer fully and approx 1 -2 mm buffer is above the gel shall I have to reduce it.

Measure the distance in cm between the electrodes. You can run agarose gels at up to 10volts per cm so you may be able to turn the voltage up. I think that 1-2 mm of buffer above the gel is fine. Make sure that the concentration of the TBE in the gel is the same as in the running buffer. Make a note of the current. When you have run and viewed the gel try moving the gel into another gel tank and run it briefly at the same voltage. If the 2 currents are very different then you have either a buffer concentration problem or an equipment problem