Can you describe your process (do you oxygenate or aerate the starters prior to using the shaker, how vigorous is the agitation, do you decant the supernatant, etc.)?
I get White Labs yeast and make a one liter starter with 1.040 canned wort. I decant, add glycerin to the yeast and place in sterile Falcon Centrifuge Tubes 50ml. These go into a Styrofoam ice chest into the freezer. When I decide to brew I pull out a stored yeast from the freezer and thaw. In the Fernbach I add 1/2 quart 1.040 starter, air pump oxygenate with s/s stone till foamed to the top, then add yeast. Run on the shaker 36-48 hours at 100-125 rpm. Pop the flask into refrigerator at 36 degrees. This flask may sit in the refrigerator for a few days. On brew day I decant this flask, add 1.070 wort, add oxygen (not air this time), until foamed to the top, and return to the shaker at 100-125 rpm. By the time I am done brewing the yeast starter is at full krausen and ready to be pitched. Please note I only make big Belgian style beers and this is my technique for big beers >1.080 OG. If I was making small beers, (haven't done that in many years), I would not do #2 step up.