Every airliner notices this when climbing, you get closer to the Mmo speed (maximum Mach number for operating) while at the same time the IAS decreases and you get closer to a stall due to decreasing airdensity. At some point you cannot go faster due to maximum Mach while you also cannot go slower due to minimum IAS to avoid a stall. At that point you cannot climb higher even if your engines have the power.