"While there are different reasons that blood flow may be impeded, there are also ways to try to correct the issue. The simplest effort would be to change the position of the needle, either by moving it forward slightly or pulling it back slightly, if it is suspected to have penetrated too far. There are also times that a vacutainer may not be sealed correctly, which may prevent blood from being pulled into the tube. Trying a new tube may resolve this issue. Also, if a tourniquet is too tight, it may suppress blood flow. We always want to try to resolve the issue without having to do a second puncture, but sometimes that is the only option left."