Glock magazines have very stiff springs when new and may be extremely difficult to load to full capacity. It may be necessary to use a Glock magazine loader (generally included in the Glock case with the handgun) to load the magazine to capacity or near capacity.
If the last round appears impossible to load, for example if the magazine should hold 15 rounds and it will only accept 14, then leave it loaded with 14 rounds for several days to allow the spring's tension to decrease. After a short time, the magazine should then accept its full capacity, but it may still be necessary to use the magazine loader. After several cycles of full capacity through empty, the magazine will be easier to load.