Take for example a 737. Would the pilot just lower the nose to pick up airspeed, then raise it back up once they’re satisfied? Or maybe retract the flaps a bit? Assuming the throttle is at 94% N1, would applying full throttle at that point even stop the decline in speed?
The first thing I would do is advance thrust to maximum available thrust, make sure both engines are indeed operating. Next I'd check is if the speedbrake is stowed.
I would not consider retracting flaps because when you are too slow to climb you are definitely too slow to retract flaps... could be deadly.
Normally this kind of situation can't happen unless you have at least one engine failure and the remaining engine not working at full thrust or the wings are contaminated with ice or snow. The aircraft is perfectly capable of climbing on one engine, so if you have two engines on max takeoff power and are still not climbing then something is seriously wrong. In case of ice/snow on the wings, well there is not much you can do about that in the air, put on wing anti ice and hope that it is quick enough to clear the wings of any contamination, this comes at a penalty of losing precious thrust because of the bleed air extraction. This situation is usually avoided by removing snow and ice from the wings whilst still on the ground - before taking off.
This sort of scenario happens from time to time. Before a flight the crew will calculate the flap setting and speed at which the aircraft rotates based on aircraft weight, the weather conditions, and runway length. If any of these are entered incorrectly, the aircraft could rotate too soon and not lift off when expected.
To decrease wear on the engines, modern jet airliners will often use less than full takeoff thrust. If the crew finds the aircraft is not accelerating or lifting off as expected, they can increase the thrust to the full takeoff setting. This is probably the first thing to check as it's a quick and easy step that would help get the aircraft up to the necessary speed and in the air faster.
It's very unlikely that a crew would decrease flaps in this situation, as this would decrease the lift, and thus take even longer for the aircraft to lift off. There have been several crashes where insufficient flaps were suspected as a contributing factor, causing the airplane to not lift off as expected and making it more difficult to control at low speeds. The crew might actually increase flaps to lower the speed needed to lift off.
The crew may also decide to to reject the takeoff. By the time an aircraft is rotating, there may not be enough runway left to stop safely, so the crew must make a quick decision as to what the safest option is.