Sodium Vapor Lamps

<...What could be the reason?...>

One possibility is an open circuit in the wiring between the foot of the pole and the top of it. Continuity check, perhaps?

<...Is it necessary that ballast be near to lamp?...>

No, it isn't.

No wires are ok. We have installed ballast at top near lamp. Now light is on. We repeatedly checked this. Something with ballast and light distance?

Voltage drop?

Most HID ballast supply voltage to the lamp at a very high frequency. Inductance in the wire because of the length may be the problem.

The problem probably lies with the length of cable between the igniter and the bulb. Cable capacitance attenuates that ignition pulse, and what gets to the bulb is not strong enough to ignite it.

The solution is to use the type of igniter that is designated 'long range' or similar by the manufacturer. These are designed to work with a long cable and will ignite the lamps.

Considering that your ballasts probably need replacement at 1/20th the rate of the bulbs, I don't see what this is gaining for you.

Be that as it may, I designed MH light towers for the construction industry for a while. Because these were on the end of a relatively flexible telescoping pole and subject to a lot of vibration as they were driven around, we put the ballasts on the trailer and cabled up to the fixtures. We originally used magnetic ballasts and had no problems, but after I left they tried using new electronic energy saving ballasts and suffered all kinds of issues. The newer electronic ballasts are essentially SMPS systems and are much more susceptible to voltage issues, i.e. cable resistance due to distance and/or mutual inductance at the high frequency pulse rate as mentioned by ozzb. The ballast mfrs stabilized the front end to compensate for line supply issues, but they never designed them to deal with voltage drop issues on the output side because the intended use was in close proximity to the lamp.

So if you can find older magnetic (inductive) ballasts, switch to those. If yours are magnetic or you can't find them, try increasing the wire size going to the lamps. It won't help with the mutual inductance issue, but it will reduce the wire resistance.