I found that my 2009 Bobcat CT235 was also hard to start when plugged in.
In normal warm situations you turn the key to the right, the glow plug indicater lights, you wait 20 seconds it goes out and you then start the tractor. I found in cold weather and when plugged in and when you turn the key the the right the glow plug light does not illuminate indicating the tractor engine is warm and you can start the tractor. In this instance I found the tractor very hard to start but once it did you could tell the block heater was working because the engine was in fact warm and running smooth.
To remedy my situation when the tractor is plugged in I turn the ignition key to the left to manually start the glow plugs hold it for 20 seconds and the tractor fires up immediatly.
My thought it this, the sensor that tells the glow plugs to come on must be situated near the block heater or somewhere not near the glow plugs so the tractor engine oil is warm but you still have to heat the glow plugs to ensure immediate combustion to start the tractor.
Does any of this make sense? In any event that is what I do all the time. When the tractor is plugged in I manually fire the glow plugs for 20 seconds an immediate start.
Fred