This is a difficult topic of discussion, because it could be any number of reasons.
Firstly, just because you don't 'feel' it in your back at the time or doing the rep, doesn't mean that your back is not 'activating properly'. If i'm being pedantic, if your back was not 'activating properly' your body would be incapable of doing a pull up.
Most likely it's down to your technique. For example, How do you perform pull ups? is your grip close? shoudler width? wide?. Is your grip overhand? underhand? neutral? If you wanted to specifically target your lats, i would tell you to do a wide grip pullup(overhand).
You stated that you 'feel it' more in your shoulders and chest, this doesn't mean that they are doing the bulk of the work, or any. It could be the case of them being tight, and you hanging causing these muscles to stretch. Which is where the sensation could be coming from.
I wouldn't worry too much about what you can 'feel' during the reps themselves. In my opinion a better measurement would be something more tangible, like can you do 2 more reps than last month? If that's the case then you've gained some strength in your back, as that is the primary mover of the pull up.
To give another example, if you were to perform a barbell squat, you'd most likely feel soreness in your quads the next day. But not so much soreness in your glutes or hamstrings. That doesn't mean that your glutes or hamstrings weren't activating properly.
Another thing worth mentioning is what your rep and set scheme look like for pull ups in your workout and how often do you do this workout? For example, if you were to just do 10 pull ups once in between your tennis matches, one per week. It would not be surprising that you're not sore. Whereas if you were to do 10 sets of 10 reps twice a day i imagine your back would permanetly be sore. (please don't do that it was hyperbole).
As an anecdotal note, i rarely feel a 'back pump' when i'm doing pullups. I usually feel it more the next day when i'm sore.
