I recall a radio report where a news reporter asking a US military officer about a given war, I want to say it was the Syrian civil war but I don't recall exactly.

The reporter referenced that one side, who had been relatively well equipped and trained compared to their opponent was no longer making much progress. This was somewhat confusing on the surface.

The officer explained that at a given scale every army is on the clock as far as how well it can perform with regards to its training. There's just a limited amount of time they have to get the job done if X% of their forces are engaged on the front line.

Eventually due to casualties or just cycling out trained troops due to service time the strategies and training and discipline break down and it is very difficult to recover without large scale training of troops that most countries can't afford / do (something akin to massive WWII mobilization and training systems).