Thank you, all, for these comments.

I found I was able to cut the pills into a 2/3 piece and a 1/3 piece (approximately). I'd been taking the 2/3 piece (or two 1/3 pieces) for ovre a week. This still didn't provide the same response as the two 50mg pills.

Then I started working a third explanation this morning -- oral absorption. The 50mg pills would partially dissolve in my mouth before I could swallow them. But the new 150mg pills have a smooth coating; they go down without a hitch.

The 50mg pills do not have a coating. They remind me of the old-fashioned aspirin tablets (before enteric coatings became popular). The pill(s) would get stuck to my tongue. (I take all five pills at once.) I'd swallow all the other pills, but then have to take more water to swallow the 50mg Nuvigil.

So this morning I held the 2/3 pill in my mouth until I started to taste it. Only then I swallowed it. It's only the first day of trying this, but I seemed a little more alert after that, even though I was extra groggy. I think "oral absorption" might explain the effects.