Ask Mr. Smarty Plants

According to this USDA Plant Profile on Mahonia aquifolium (hollyleaved barberry), it does not grow in Illinois. Of course, you can purchase any plant and plant it wherever you want to; that doesn't mean the plant is going to like it.

From a website from Ohio State University we extracted this information about Oregon Grape Holly; highlighting some areas we thought were pertinent to your problem:

"Culture:

• partial sun to full shade
• does best in partial shade in moist, rich, well-drained, acidic soils; not especially tolerant of alkaline pH soils, poor soils, compacted soils, clay soils, wet soils, heat, full sun, exposed sites, or at windswept corners of foundations
• propagated by seeds, stem cuttings, or division of multi-stemmed parent plants
• Barberry Family, with several minor disease and pest problems, but most often troubled by improper placement in the landscape (see above), resulting in stunting, foliage chlorosis, and/or foliage Winterburn
• moderately available, in ball and burlap or container form
• needs a protected site in Winter against prevailing winds, channeling winds, and direct sunshine (as do all broad-leaved evergreens to avoid Winter foliage dessication)

From this website, Paghat's Garden, we learned that mahonias do not fruit well unless they are able to cross-pollinate with other mahonias. If you have just the one bush, and no others are growing in your neighborhood, that could explain the no fruit, but not necessarily the no flowers. Even if there are mahonias around close, they are bee-pollinated, and there has been a drastic shortage of bees all over the world in recent years.

Frankly, we have no ready answer as to why your shrub is neither flowering nor fruiting; we can only follow clues that point to it not flourishing where it is.  We suspect climate conditions, as it is native to Oregon and the northwest, flourishing in woodlands. Another culprit could be the soil you have; in a woodlands, with generations of fallen leaves, the soils are acidic, which this plant requires. We don't know of any soil amendment, fertilizer or spray that would correct those problems sufficiently to make the plant feel at home.