If you see the classic ski-slope curve, then there is either a fault with the sensor, or the sensor experienced a shock/transient during the measurement process. The transient may be mechanical (the sensor was bumped, or experienced very high vibration), thermal (the sensor was placed on a very hot or cold surface), or electrical.
The spectrum will have a ski-slope and a raised noise-floor at the high frequency end of the spectrum if the transducer becomes "saturated". This typically occurs when there is a source of high vibration at high frequency such as cavitation.
Make sure the settling time is long enough, especially if you are using a low cost sensor or a sensor with greater than 100 mV/g sensitivity.
If you are testing a hot machine, you may need to leave the sensor on the machine for 5 minutes to allow it to heat up.Test other hot machines in a sequence so the sensor stays hot.
Look in the time waveform; you may see a spike or a "ring down" (the dynamic vibration will have a changing DC offset) or you may see signs of cavitation.
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