Can be several different things...
Most common are...
A loose connection or corroded ground on a bat cable or at engine connection.
A voltage drop from the ign switch not supplying enough voltage to trigger the solenoid.
Moisture inside the starter reaking havoc.
The above are the most common. If it is a starter problem and not a supply problem IMI may give some warranty consideration on it. Just give us a call if you need some assistance in that regard. We use a lot of IMI's and have very few problems with them. But check all the electrical connections well first before you blame the starter. Many times these things are not the starter but a wiring deterioration problem.