I've rebuilt my 280 about 3 times in the last 10 years. There is not a single part left from the original unit! It came with the house 12 years ago and was fairly worn out when I got it.
Couple things to check:
You can adjust the rear wheel forward to engage the idler wheel (small one in the middle), but by the looks of it, it could be the swing axle that the idler wheel is mounted to. There is a T-nut and spring behind it together with a plastic guide. All these parts can be had for about $10-$15 on-line - Swing Axle Kit C-36.
Does the idler wheel move and spring back?
And, like gonzodog said, check for wear on the teeth inside the drive wheels. If the shaft is okay, but the front wheel is worn, it might have been adjusted too tightly against the shaft. Quick fix for this is swap the front wheel with the back wheel (there is nothing touching the teeth on the back wheel - so no wear.
If you do this, 2 things to be aware of: the bearings WILL fall apart if you're not careful - like BB's bouncing around. Also, very easy to "strip" the slotted head - use a tight-fitting screw driver for removing. Pool stores want like $10/ea for the bearings (there are 2 on each wheel); you can find those on-line for $5-6, sometimes less if you buy in quantities of 4-8.
EDIT: During my last rebuild, the wheels come already mounted with tires, bearings and adjustment plate. If the wheel gears are significantly worn, just opt for a tune-up kit like this: ($145 online)
View attachment 47639
The other thing - when mounting the wheels and adjusting to the axle you DO NOT WANT a "tight" fit, rather a 2/3 to 3/4 "engagement" of the drive axle to the teeth. It's a little tough to get this right, but you'll get the hang of it.
Oh, and be careful if adjusting the wheels to not loose the little copper/brass star washers under the adjusting screws. I did and replaced with stainless steel ones from the hardware store. That'll work - for maybe a week or two and then the wheel falls off. Been there, done that. Something about the alloys and water chemistry and galvanic action or some such. There is a reason they are not Stainless!
Let us know if any of this works.