It was not until the early evening of 16 September that Serbian troops finally reached the summit of the Kozyak, having had attack after attack thrown back. Even then they came across a German battalion on the northern slopes, covering the withdrawal of the Bulgarian defenders, so keeping up momentum was difficult.
Alan Palmer describes General Franchet d’Espèrey’s next move:
By the morning of September 17 the reports reaching [him] were favourable. A salient six miles deep and 20 miles broad had been driven into the Bulgarian line around the Dobropolje, and the Germans were withdrawing units farther to the west in order to prevent them from being cut off in the loop of the River Crna. It was time to put pressure on the Bulgars in the Dojran sector as well; and Franchet d’Espèrey ordered Milne to launch the Anglo-Greek offensive at dawn on the following day, September 18.
This map from Alan Palmer’s The Gardeners of Salonika (1965) shows the tremendous progress of the Serbian and French assaults from the 15th to the 17th:
Palmer, Alan (197?), ‘Defeat of Bulgaria – The Central Powers Begin to Crack’, published in History of the First World War No. 107 by Purnell for BPC Publishing Ltd, London, in cooperation with the Imperial War Museum.