There are two different chemical shift scales in common usage for 15N NMR. IUPAC (international union of pure and appliced chemistry) recommends nitromethane for the chemical shift reference (0 ppm). Our Bruker instruments use the recommended secondary reference (standard for biological samples), liquid ammonia. When setting up the center frequency of your spectrum (o1p) be sure to use the correct chemical shift scale for Bruker.

Selected Values of 15N Chemical Shifts

(F.A.Bovey, Nuclear Magnetic Resonance Spectroscopy, 2nd edition,
Academic Press, Inc., 1988, p.461)
compound IUPAC scale "Bruker scale"
CH3NO2 (neat) 0.000 376.86a
NH3 (neat) -381.9 0
NH3 (neat; -50 °C) -376.4b 0a
NH3 (neat; ambient) -380.4 (27 °C)b -3.37 (25 °C)a
(H2N)2CO (urea; DMSO) -302.6 73.4c
H—C=O·NH2 (formamide; neat) -267.8 b 108.2c
a A.McDermott & Z.Gu, Carbon & Nitrogen Chemical Shifts of Solid State Enzymes, Encylopedia of Nuclear Magnetic Resonance, Vol. 2, p.1137
(Chem-Bio library REF 543.429 ENC)
b J. Mason, Nitrogen NMR, Encylopedia of Nuclear Magnetic Resonance, Vol. 5, p.3222
c adding 376 to the value in Bovey.

Recommended reading:
Nitrogen NMR, Hebrew University