My dear little Tilly came home about 3weeks ago too!
At first she was very shy, didnt want to play at all, and slept the most of the day (actually ALL day!). She also didnt want to eat at all.
A fiew days ago she wouldnt eat for 3 days, and I took her to the vet. She had a cold! And the most likely reason she didnt eat was because of a sore throat. We got her injections and she is on medication.
Now she is starting to feel better, and atlast I can see her personality comming out. She is so playfull, but not over-energetic, she has her naps, always wants to be close to me. And she is eating better (but still a picky eater). In other words she is just the Perfect Yorkie!
I think after all this time, she is actually adapting to our house, she is getting used to me, (as Im getting used to her) and now she actually isnt that shy anymore, and loves to play with me.
My main point is take him to the vet (if you havent done so) and just do a general check-up. (especially check that he is not over-weight). If nothing is wrong with him, just give him heaps of love and kisses. Try to play with him but be very gentle not to scare him off. He might just be a very quiet Yorkie, but he will loosen up, and I believe ALL dogs loves to play, even if it is just 5min a day. You will get to know each other, and you will get to know his likes and dislikes (Tilly for instance only likes soft toys).
Always remember: good things takes time
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