It Was an Uphill Battle
The Allied Forces planned their offensive on the Macedonian Front with 619,000 men, versus the 600,000 men of the Central Powers. They had the advantage of numbers, but they were on the offensive and they needed all the men that they could get. They had the advantage of artillery but in some ways the artillery was a burden as well. But even with the advantage of men, the Allied forces still faced a very big problem in the form of large mountains.
The front at the Battle of Dobra Polje was 15 kilometers (9.3 miles) long, between the Sušnica and Lešnica rivers. The mountains between the rivers were between 1,400 and 1,800 meters (4,500 to 6,000 feet). The mountains themselves were not easy climbs, and there were numerous steep slopes that ended at ridges that were fortified by the Bulgarians.
The addition of trenches meant that there was no way for wheeled vehicles to make it up the mountain. There were some areas that had previously only been accessible by mules and not made to transport vehicles or heavy artillery. Even today, the areas pushed forward by the Allies are only reached with four-wheel drive and a guide that knows the correct paths. While not the most ideal place for a battle, it was the best option open to the Allies because the rest of the Macedonian front was even more mountainous.
If the mountain itself was not enough, there was the problem of the Bulgarians and the German 11th Army. They had had years to prepare with deep trenches and fortifications that allowed them to not only have the advantage of high ground but make it hard for the Allies to break down their fortifications. The Serbians refused to let anything stop them from liberating their home, and they pushed forward eventually managing to get their artillery up the mountain with them.