In a previous post I emphasized the importance of cleaning the outside of your NMR tube before putting samples in the NMR magnet. The "stuff" from your fingers (on the outside of your NMR tube) accumulates on the inside of the NMR probe coil inserts and can cause spinning problems, shimming problems and problems with inserting or ejecting samples. Furthermore, the accumulation of "stuff" causes a significant background signal. The figure below shows the top of an NMR probe before and after cleaning.
The probe was in use for several months before cleaning. Notice the sticky black "stuff" present in the photo on the left. Please take care in cleaning your NMR tubes before putting them in the spectrometer.