Generally pilots may want to maintain their altitude if they find themselves to be too low on an approach if above 1000ft. Once you’re within that 1000ft - 0ft AGL, you’re in a phase of flight where you need to be what’s called “Stabilized”. Stabilized criteria includes but is not limited to the aircraft being configured (gear & flaps), on speed, on glideslope and within the localizer.
If they’re below or within this 1000ft region, and an aircraft deviates from the glideslope most aircraft operators will permit the crew one attempt to regain that glideslope. If the crew fails to maintain the glideslope on the second try, some if not most airline operators require a go around. Its fine for there to be slight deviations but the flying pilot needs to state his intentions to the other crew member that he is correcting.
The moral of the story is this. Regarding the airlines, we have very strict procedures that offer very little to no leeway in a situation like this. Stabilized criteria is a must once they hit that 1000ft AGL target. Anything that isn’t near perfect from there to the ground will require a go around to be made. If an aircraft is too low on approach after being cleared for that approach, the crew did something wrong which would likely warrant a go around. Getting behind the aircraft and not staying ahead of it is usually the common reason for such. Pilots not paying attention or being distracted. Mismanaging aircraft energy or altitude.
Maybe this may clarify. 🙂