It will be one or both pin receiving housing/latches out of alignment. Just adjust them so pin is centred.

Sometimes, I push the button, hear the noise, get out back, and it's still locked. Then I do it again and it unlocks. I'm in the habit of pushing the button twice now and it's usually good. That, and I hold the button longer

Also, the rubber seal is often too sticky; depends on the weather I guess

+1 @PEvans. Even when these cars were new the hatch didn’t spring fully open when the release button was pressed, it just unlatched it so you could open it by hand. If it’s not unlatching you can adjust the pins slightly until it does but be careful as those pins tend to corrode and seize up.

I replaced my pins with stainless ones a while ago and that prevents them seizing but I still seem to have to adjust them a couple of times a year. I’m not certain if it’s wear or the weather that causes them to stick, probably a bit of both.