Gear shift stuck

I have a '04 Sienna Le. At times, I couldn't get the gear to shift into Drive when the car is parked. It's stuck. Took it to Toyota Dealer but the problem comes and goes. Unfortunately, it didn't happen at the Dealer so they couldn't fix the problem. It's really frustrating. Sometimes, it takes me 15+ minutes just to get it into Drive. Anybody have any similar problem or any idea what's causing this?

Your feedback will be greatly appreciated.

When gearshift is stuck, is the van on a hill or grade? If so, is parking brake on?

No, it's parked in a leveled parking. The brake is never on when it happens.

Try applying parking brake while car is still in D, then move to P. See if gearshift still still sticks.

Thanks! I'll try that.