At Samizdata Patrick Crozier points out some things that are necessary to understand why it took so long (relatively speaking) for the “new army” (and the other freshly raised Dominion and Empire forces) to begin winning battles:
First of all, Britain was fighting a war in Western Europe against a large, well-equipped and tactically skillful enemy. That is a recipe for a bloodbath. Britain repeated the exercise twice in the Second World War (May 1940 and June 1944 onwards). They were bloodbaths too. We tend to forget that fact because overall the numbers killed in the Second World War were much lower than than the First and because they achieved a succession of clear victories.
Secondly, Britain began the war with a small army. To make a worthwhile contribution Britain was going to have to raise and train a large army. Soldiering, like any other job, is one where experience counts. Anyone who is familiar with the rapid expansion of an organisation will know that this is a recipe for confusion and chaos. In the case of the British army the inexperience existed at all levels. Corporals were doing the jobs of Sergeant Majors, Captains doing the jobs of Colonels and Colonels doing the jobs of Generals. Haig himself (according to Gary Sheffield) was doing jobs that would be carried out by three men in the Second World War. Talking of the Second World War, it is worth pointing out that it took three years for the British to achieve an offensive victory (Alamein) over the Germans which is much the same as the First (Vimy).
Thirdly, Britain began the war with a small arms industry. Expanding that involved all the problems mentioned above plus the difficulty in building and equipping the factories. It comes as no surprise that many of the shells fired at the Somme were duds and even if they were working they were often of the wrong type: too much shrapnel, not enough high explosive.
Canada’s army in both 1914 and 1939 was a tiny cadre of what the nation would eventually raise, train, equip, and send to foreign shores. The amazing thing is that they managed to become a valuable fighting force to the British army in particular and the allied cause in general from such a tiny, unmilitarized population. Britain’s highly competent regular army was effectively expended to gain enough time for the volunteer forces to train and organize, and even then the early going was far bloodier at least in part because the troops had still not been sufficiently trained and had to be lead in ways that exposed them to higher risks of casualties whenever they were on the attack.