Simulation problems may come from default simulator absolute tolerances that are changable if necessary. In real world, small currents of course can be difficult to measure. Cable capacitance may be a serious problem, also mechanic interferencs causing currents from intrinsic isolator charges or piezoelectric effects. Electrometer amplifiers sometimes use triaxial cables with an active driven guard potential to reduce cable capacitance influence, but the best measure is to move the amplifier to the signal rather than using long cables.