In my work, I measure the mechanical properties of PAAm hydrogel by PFQNM mode using PFQNM-LC_A_CAL probe from Bruker in liquid. When the systems approachs on the surface of the sample. The SUM decreases fast from 1V to 0.5V and the horizotal bar increase from 0 up to 2.5V. Thus The system refuses to engage. This the first time I get this problem. I attache the picture below. Could you give me some advices ?