Thicker skins will give you a bit of impact resilience though, never a bad thing on an under-body item like a diffuser.
I have seen this exact problem before. It is caused by not having a vacuum port at the last point of infusion. There is always some residual air in your preform even though it is under vacuum. This air gets displaced as resin infuses the preform. But, if a section of dry preform gets surrounded by resin so that there is no longer a path for the air to get to the vacuum port, the air can no longer escape. This residual air will get compressed until it equals the resin pressure and then no more resin flow will occur. To prevent this problem you have to control the speed an direction of the resin flow front so that the vacuum ports are at the last section of preform to get in fused.
You can do an infusion by just bagging over the dry areas but, again, pay attention to the flow front. Infuse from one side to the other and avoid any race-tracking of resin.