Problem concentrating during Vipassana meditation
When you breath be mindful of the start to end of the in and out breath. For each breath in and out breath be mindful of whether your mind wandered away during this time. Also note the length of the breath in and out breath. 

When scanning the body keep your concentration anchored on the breath and sensation on the base of the nose as well. If your mind wanders away bring it back to Anapana and then continue the scanning. For each in and out breath see if your mind has wondered away for that period. Also note the sensations that arise when your mind wanders to an alternative object and the sensation when it is with the chosen object.

In some parts stay for a while (but not too long) also focusing on the breath. Sometimes this helps sharpen the mind to see clearly the sensation arising and passing away in the particular section of your body.