Plotting a well defined function displays nothing for two-thirds of the range required


The plot in question concerns the second derivative of an inverse Laplace transform (ILT) of a function with five parameters. Here is the ILT

ClearAll["Global`*"] prod = (s - cr1) (s - cr2) (s - cr3) (s - cr4); LW = (1 + s)^2/(si prod); Print["symbolic W'=", Wp = D[InverseLaplaceTransform[LW, s, x], x]] 

Four parameters are functions of the fifth parameter "si", defined as the roots of a fourth order equation

cr = {cr1, cr2, cr3, cr4} =     s /. Solve[si s^2 + 107 s/5 + 10 ((1 + s)^(-2) - 1) - 1/10 == 0,       s]; 

Plotting the first derivative of the ILT takes .64

lx = 13; Timing[  pd = Plot[Evaluate[Wp /. si -> 1], {x, 0, lx},     PlotRange -> {{0, lx}, {0.0225, .0275}}]] 

Plotting of the second derivative of the ILT takes 14.84 and displays nothing for two-thirds of the range lx

    Wd = D[Wp, x]; Timing[Plot[(Wd /. si -> 1), {x, 0, lx},   PlotRange -> {{0, lx}, {-0.002, .002}}]]