1. You're not eating or sleeping enough for the weight to go up
Boring, and the most common one. If bodyweight hasn't moved in two months and you're trying to add weight to a lift every week, that's the stall. Fix: eat an extra 200–300 calories a day for four weeks, mostly carbs around training, and get seven hours' sleep. If the bench moves, you've found it.
2. You're doing too little, or too much, of it
Bench needs practice: twice a week minimum for most people, three is often better. Six to twelve hard working sets a week is the range that works for the majority. Under six, add a session. Over sixteen and you're always sore, cut back to ten and watch it climb.
3. Your progression rule has run out
Adding 2.5 kg every session works for a few months and then it doesn't; the lift has outgrown linear progression. Switch to reps first: work at a weight in a 6–8 rep range, add reps each week, and only add weight when every set hits 8. Or take 10% off and build back up over four weeks, which almost always lands you above where you stalled. What matters is that you change the rule, not that you push harder on the old one.
4. Something is weak in the movement
Last, not first, because most stalls aren't technique. If the bar stops a few inches off your chest, add paused bench (2-count pause, sets of 3–5). If it stops near lockout, add close-grip bench or dips. If your shoulders are wrecked, your upper back is probably the issue: double your rowing volume for six weeks.
How to know which one it is
You need your logs. If your rep counts have been drifting down at the same weight, it's 1 or 2. If you've repeated the same weight for three sessions with no change, it's 3. If the bar always stops in the same place, it's 4. Without a log you're guessing, and guessing is how people stall for a year.