Question: I have a manuscript under review at Nature Scientific Reports. The editorial system shows eleven reviewers invited but only one has accepted. Is this a problem? Is it normal?

Answer: 
I think it really depends on the field. Having published a lot of work in Systems Biology intersect biology, comp sci, math, modelling, ... I found it really hard to get reviewers for our papers. And once you have reviewers they often evaluate the paper very one sided (e.g. the math is trivial, no new concept in comp bio, limited biological novelty, ...). So yes, it is a problem, but you are asking the wrong question: I think you want to know what is the cause of it and can it be fixed?
Yes, it indicates a problem:

* It could be that your paper is uninteresting, such that most people don't really want to spend the time to review it.
* Or it could mean that the editor is "missing" and identifying people who aren't in the same field as your manuscript.

There's nothing to do except wait, though.