My Sportage did this about once a year, every year. I thought about getting locking hubs but after I took it to the dealer to repair it a couple times, the problem was always the air hoses that go to the back of the front wheels. They would both crack and break at each end of the hose.
The next time it would not engage, I just turned the front wheels so I could see the back side and sure enough, there is a air line running from a nipple coming out of the inside of the fender well to a nipple on the back side of the wheel drum. Each end of the air hose was broke. I could instantly see a very poor design. The factory air line has a built-in rubber gromet in the middle of the line that goes in a metal bracket that keeps the line in place. But there is no slack and is way too tight when the wheels are turned. This poor design puts too much pressure on the hoses where they hook to the nipples every time you turn the steering wheel just by normal driving. The lines always break right where they attatch to the nipple fittings on each end. Both ends break on both front wheels.
A very simple fix that works. Don't buy the factory assmbly but take the old air hose to a auto parts store and just match up the right inner size with a piece of new hose that comes on rolls. Only cut a piece that is about 6 inches longer and don't hook the hose to the metal securing bracket but just run the line through it. Put a loose fitting zip tie in it's place and leave some slack. This gives the hoses plenty of room to move as the wheel turns back and fourth.
This only takes a few minutes to fix. You don't have to jack up the car or even open the hood, but just turn the wheels sharp to access the lines.
They never break on me now and my 4wd always works. Hope this helps.