Withdraw submission due to lack of referees
Some reviewers reply to invitations slowly or not at all.  Worse, some reviewers initially promise to review, then later change their mind (there can be good¹ or bad reasons for this).  Worst, some initially promise to review but are then never heard from again.  The job of the (associate) editor is to somehow handle all of this, and since reviews are voluntary there's very little the editor can do.

I was recently a reviewer on a paper that was slow.  The review of my co-reviewer never materialised, and other invitees either ignored the invitation or declined.  After several months, the editor, fed up, sent out 17 invitations in a single day.  6 declined the same day.  6 declined up to 2 weeks later.  4 never replied.  1 accepted and delivered the review.  Ultimately, the discussion phase was extended almost 3 months beyond what was originally planned.

Bottom line: don't read too much into the review being slow.  Although some publishers communicate some details to authors, most don't.  Just wait.  Solving this problem is the editor's responsibility, not yours.



¹A colleague agreed to review a paper (his first) based on the abstract.  When he received it it was almost 100 pages long.  I think he sent it back with a short note along the lines of I can't review this, please shorten it or write a book instead.  I don't know what happened to the paper in the end.