Amino group (-NH-) has an exchangeable proton. In NMR, exchangeable protons give broad signals and if the concentration is low then you may not see the peak in the NMR spectrum.

Exchangeable protons like -OH and -NH could give broad signals or (almost) completely disappear in the NMR spectra (especially in solvents in which this equilibrium is favored). Higher concentration could help to see these signals.

i am also in the favor of above discussion, higher concentration could help to see the peaks.

What's the resolution of the NMR instrument u analysed? The NH peak wouldn't show up in lower resolution instrument.

Use d6-DMSO for NMR experiments where you wish to observe a exchangeable proton. Definitely don't use D2O or CD3OD. CDCl3 will also lead to replacement of exchangeable protons with deuterium due to residual DCl,  though this is much slower.

Phillip Leigh Van der Peet Thank you for your response! What could you tell on capacity of C6D6 or C7D8 to exchange protons with (CH3)2NH or t-BuNH2?

Neither C6D6 nor C7D8 have the capacity exchange protons for deuterium. But as the others point out, you may find your NH peaks are quite broad, particularly if tautomeric forms are a possibility.

1-Ensure that the sample is not heated during the analysis, because heating leads to loss of signal.

2- Try to analyze by a device with the highest accuracy.

3- If the interaction includes several steps, the peak may appear in the following steps.