It is hard to deal with not knowing what you are doing wrong in an experiment. Not because you have to do it again, that is the least of the worries, but because if you don't know what is wrong you can't fix it. 

Though this is also a great way to challenge your ingenuity and patience. 

Cleanliness may be the key! I don't know, I always think I am being extra careful in my cleanliness. 

Being extra careful may be the key! I don't know, I always think I am being extra careful.  Well, working with microscopic organisms is really amazing.  You have to be a perfectionist in your experiments.  At least cells and viruses don't have feelings.  Or do they? Working with people is way harder to deal with.