Why won't my car go into park sometimes?

Because it is a crappy automatic and you should be driving an manual.

Serious answer:

It could just be the linkage it dirty or loose. Or it could be a problem with the transmission. Most new auto trans are electronically controlled so it could be some sort of malfunction in the computer refusing to engage park. But if the car drives fine otherwise, it's probably the linkage. Your dealership is stupid if they didn't catch this, so go somewhere else to fix it.

Car Wont Go Into Park

There is a safety device installed in the shifting mechanism that it preventing it from going into park. It is there to prevent the accident of stopping and turning off your car while in gear and you leaving the vehicle and it rolling away or a child from putting it in gear without stepping on the break first. Take it to you mechanic to have it looked at.

PS try stepping on your break and see if you can then put it in park.

Your transmission shifter linkage is out of whack. Easily adjusted at the dealer. Takes about half an hour.

Hold down the break pedal, and then shift the knob into park

then you can remove your key

probably the linkage needs adjusted.