Disappearing Tulips

The tulips under the river birch either are disappearing or divided and not producing flowers. Here is a shot from this year. There are only two small white tulips here.

Where as last year, we had a good clump of ten or more big tulips. See picture below from 2008. Perhaps they are not getting enough light after blooming, or have split and will come back in full force next year. I will have to keep an eye on this site, as it may not be a good place for them.