I have a Kenmore Heavy Duty about 10 years old. It doesn't dry the clothes completely.
I have to put the clothes through 2 cycles to completely dry them. Already checked the air vent, it works fine. What else could the problem be?

Have you checked for clogs in the venting and cleaned it out well. Als discarded the lent from the lent tray? Check the distance from the wall and make sure the hose isn't being cramped. Also clear away anything that may be blocking it outside. Sometimes there is lint on that screen as well, or trees etc. outside. Good Luck

What else could the problem be?

A defective or burned out heating element.