Compare with a fresh standard, not with a sample of unknown history. Low peaks come from a partial injection (blocked syringe, leaking needle seat, air in the draw), a standard that has degraded, the wrong wavelength, a weak detector lamp, or a column that is retaining or degrading the analyte.
Change one item. A new standard that restores the height points to the sample. A new lamp, or a failed lamp-energy check, points to the detector. A height that is low only on some peaks points to the chemistry or the column, not to the injection volume.
Call Mahut if the autosampler errors, if lamp energy is failed, or if the loss appeared suddenly on every peak including a fresh standard.