It's luck, where and whether they like you.
In southern Tarangire the area is known for them and you'll see those black/blue triangles with repellent to ward them away. Also saw those triangles various places in the Serengeti (north especially). Otherwise either country, it's best to avoid blue/black and hope for the best especially when transiting thru woodland areas.
On my various visits, I seem to get away with being attacked, even if I am in colors that shouldn't be worn, while my guide in tan is bombarded... so beats me.
Believe in all these years, only one bite, between my fingers on drive thru Serengeti to the Western corridor. My guide was spared, but my friend was eaten alive and we were sitting next to one another... they just flew past me!!!!
This past visit we did pass some areas both in TNZ/KEN where Tse-tse abound, and again the guide got it, but his two paxs (moi and friend) missed out. When entering an area, just roll up the windows, turn on the a/c till out of there. Whatever you do if bitten, remember... do not scratch, rather apply some anti-itch creme.
I rarely apply repellent... just seems I'm not a target. Some places provide those repellent wipes, whether with or without Deet, I don't know (print too tiny to read), which I did use at a camp in Mara... on my ankles. Seems any critter that might be seeing me as a meal, goes for the ankles, nowhere else on the body.
Honestly, I rarely think of tse-tse and am properly dressed during mossie biting time - dusk/dawn.