Female JRT Puppy Biting

Question
I adopted my almost 9month old female JRT puppy(Sydney)when she was 8wks old.  She has been spayed and is doing fairly well with potty training. I am the primary care provider for Sydney, although my husband also helps with her care.  I also have a 20yo son who is home on the weekends from college.  Sydney is crate trained--she is in her crate most of the day, as both my husband and I work outside the home.  When I get home from work each day, she immediately goes potty, she eats her dinner, and then we play.  In the past couple of weeks, toward the end of our play time--or it seems when I am no longer giving her my full attention--Sydney will bite at my hand or arm.  She will bark or growl and lunge at me while doing so.  A lot of times, her tail is wagging as if she thinks we are playing, but her biting hurts.  She knows how to sit, stay, and shake--I will generally make her respond to one of these commands in order to get a treat.  I have tried to get her to stop biting by redirecting her to a toy ("where's your ball") or giving her a toy, I tell her "no bite"--sometimes the combination of this will make her stop intermittently, and I will praise her heavily ("good girl, sydney--no bite").  When I initially adopted Sydney, she was home most of the day with my son, as he was attending community college and living at home--she got a lot of attention and was not in her crate so long each day.  I know Sydney misses my son, too, since he left for school--could this be causing some of her behaviors?  Also, I have just ordered a padded harness for Sydney, as I want to start taking her on evening walks--she has done well with this thus far, but her current harness has been rubbing under her arms.  Anyway--any help you could provide would be much appreciated.  Syndey is soooo precious to me--I just want to make sure that I am doing everything to ensure that she is safe, secure, healthy, and happy!!

Answer
Hi Shellye,

Sydney sounds like a great girl.
She has definitely been going through lots of changes with your son leaving
and then having to spend so much time alone in a crate.  4 hours should be
the absolute maximum a dog should be in a crate during the day.
I have seen "bad" behavior happen in these cases from sheer boredom.
If Jack Russells don't have at least 2 good hours of hard play, they will find
another way to disperse that energy and it comes in the form of rough play or
destroying things.  If there is any way at all one of you could come home for lunch
and let her out to RUN hard for a little while, the negative behavior should subside.
Exercise is the #1 priority in keeping a jack russell.
Maybe you could hire a neighborhood kid to come in once a day to take her out?

You are doing well with the NO BITE command, but don't praise her at all, you are
confusing her and she might think you are praising her for biting.
Say NO BITE but then get up and walk away.  "Play" stops immediately when biting starts.
Don't deviate from the correction, make it the same all the time and she will learn quickly.
It's good to praise but I think you're doing it a little too soon after the biting.
Eliminate it completely and only praise after something positive, like peeing outside.

The padded harness sounds wonderful!!  Best of luck to you all!