Layer Mask Difficulty

I have been using layer masks for the last couple of months and found them very useful. However, I've hit a snag: when I use mask to get rid of something, it doesn't completely erase. My mode is normal, my opacity, fill and flow is 100%. The layer I'm trying to manipulate is not locked and I am on the mask layer. And yet I can't completely erase what I want off the layer.

In the example here, I tried to erase the wording "EXISTING DAYLILIES" (Obviously didn't work)

This problem has only popped up in the last few days, so I'm not sure what has changed.

Thank you very much!

- Fletcher