Below the shadowy surface, there may be unseen branches and logs and sharp-edged rocks, or even the rusted hull of some old sunken Chevy. Inevitably, your hook or lure will get stuck on something. You'll probably try to walk a few feet up or down the shore to try to pull it loose, but no matter how hard you tug on the line, you can't free it. The only thing to do is cut the line and start again. Don't worry about the lure you've lost.
Every writer has snags, blocks, off days.
The effective writer overcomes them. Dwelling on the snag will magnify the block, and furthermore, it's simply not true. If you fish long enough, sooner or later something will bite. It might take a few hours, days, weeks. But writing moves in cycles, like the fishing forecast bad days, good days.