TRACKER DOES NOT WORK

my First vechile was ****** in 2013 30 D4D Toyota (hijacked) they lost signal on the unit about 30min after it was taken.They found the unit 1 week later lying on the side of the road. in 2020 My Ranger was ****** on a site we were working on, The Tracking device was activated, they got a signal for 2 block then lost the trace .The unit was found near tembisa 2 weeks later, both units were taken out of the vehicle, but yet they could not find the vehicle but managed to find the tracking unit.....HOW? can you find the unit but loose signal after the vehicles were taken.