could be a few things
First, are they native pecan trees? That is, (1) were they either purchased from a nursery as "papershell" trees or have they been grafted to papershells or (2) are they either natural (native)occuring trees in that area or trees purchased as "hardy" (same as native) trees?
All papershell trees (cultivars) will produce before 20 years old, but some natives may take up to 15 -20 years or a few more prior to production. A trunk diameter of 12 inches should mean they are 20 years old or so in most cases.
Second, how close are they to any other pecan trees in the area? This is important because some cultivars ("papershells") do not self-pollinate. If both trees are the same cultivar and there are no other pollinating trees within a quarter-mile of so, they may not be getting any pollen at the appropriate time. This would also apply to two native trees that just happen to not self-pollinate, but this is not common.
Third, if they are natives, it is possible that they are just not going to produce pecans. Not too likely that neither would, though, but I have some that are 80 years old and have never produced. But again, 95% of my natives produce at least occasionally.
Fourth, is the soil shallow, dry, very poor, or severly lacking in some nutrient that you know of? (I know, tough question.)
Fifth, are you sure they are pecans??
