Tbounds

my low tire warning system keeps coming on . I fill tires with air and it goes off then in 2-3 weeks its back on . So I decided to check all 4 tires pressure , they all read 40lbs the tpms said fronts were -2lbs and right rear was -5 and left rear normal?
the monitoring system all showed 40 in all but right rear and it was 38 ?
in the past it has been the front right tire that is supposedly low?
anybody else had a problem?