You'll be only dimly aware that something is just off between you -- that something drains your energy when you two are together. One problem will be that you'll fail to set up boundaries, and your energies will be lowered as a result. Your friend could be overly sensitive to you at an emotional level, and you'll sense it when they're feeling down or hurt.
Any emotional disconnect or oversensitivity won't be your fault or theirs; it'll just be an unavoidable fact of this particular friendship. But trying to bridge the distance could prove difficult, or even futile. It might be a better idea just to stop trying. If you can accept that there's a level of disconnect and discomfort between you and just move on, you might find that it's not the big problem you'd thought it was.