We studied the nature of various types of flux density variability in pulsars using simultaneous beamformed and imaging LOFAR HBA observations at 110 - 180 MHz. Large unexplained flux density variability has previously been observed in LOFAR (beamformed) observations by Bilous et al. (2016), with fluctuations of more than an order of magnitude. In our observations we found several types of flux density variability: (1) short time scale (~minutes) simultaneous beamformed and imaging variability due to intrinsic variability in the pulsar emission, (2) long time scale (~hours - weeks) simultaneous beamformed and imaging variability likely due to refractive interstellar scintillation or variability in the intrinsic pulsar emission, and (3) beamformed flux density variability due to apparent source displacement away from the beam pointing. The size of the fluctuations of (1) and (2) are not large enough to explain the flux density variability in Bilous et al. (2016). However the fluctuations due to (3) caused drops in flux density up to 60% in our observations. Source displacements producing (3) are very likely to be due to variability in the electron density of the ionosphere which affects the ionospheric refractive index. The SKA Low-Frequency Aperture Array (LFAA) and other current and future low-frequency facilities with designs similar to LOFAR might also suffer from loss of beamformed flux density due to source displacements, therefore ionospheric activity should be monitored.