I would also suggest TT as it seems to me she is giving clear signs that she is 'saving' it for just before nap time.
We've been doing very gentle TT with DD and started when she was about 14mo as she poo'd in the bath two nights in a row, having never ever poo'd in there before, usually did it in her high chair during dinner time. I read this as a sign that something was different for her, so we bought a potty and would sit her on it before bath time every night. We have now progressed to the big girls toilet with a seat insert and generally get something almost every night.
Give it a go! What have you got to lose! =) Good luck!
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