I am attemping to plot a series of 2-D vector fields (direction fields for simple differential equations), and have run into a problem. I was able to plot one with the PlotVectorField command; however, when I change the yprime in my equation and again use the PlotVectorField command with the same parameters as the first successful one, Mathematica simple echoes back my PlotVectorField command to me instead of displaying the graphic like it did the first time. I have been unable to make any more VectorPlots since the first successful one; instead, it just echoes back what I entered without any error messages. I can still do other plots. When I enter the *exact* same thing on a friend's computer with Mathematica, it plots the next few Vector fields just fine. I have tried starting a new session to clear out any odd things I stored in memory to no avail. Any ideas are appreciated to address this thoroughly frustrating problem, thanks.