I have had different brands of tubes lose air at different rates. My wife's road bike (presta valved tubes) loses air quicker in the front tire than the rear. Neither tube has been patched, neither has any sealant, tires and rims are free of anything which would cause a leak. Isn't major enough to replace the tube which loses air more quickly, but it's noticeable after the bike might sit for a while. The only thing I can figure is that the tubes are dif.brands and lose air at dif.rates (don't remember brand names). Maybe you could try a different brand of tube?