My "Weed-eater" reel mower is 5 1/2 years old, and it shows. But if the plastic wheel treads won't dig in against the resistance of the grass, then it's over. That's probably by design to make us toss 'em sooner and buy new!
Here's a simple solution to planned obsolescence. Find a few tiny wood screws, enough to go around both wheels (4 or 5 each) and screw into treads at thickest places. Not too deep or they'll scrape metal below and slow or stop machine.