Sentence:  Following observation of the fact that the reaction rates of supposedly identical reaction mixtures prepared on the same filling manifold and exposed under identical conditions often differed by several hundred per cent , a systematic series of experiments was undertaken to see whether the <WSD>difficulty</WSD> could be ascribed to the method of preparing the chlorine , to the effects of oxygen or moisture or to the effect of surface to volume ratio in the reaction tubes . 
Meaning of the word marked with <WSD> tags:  a factor causing trouble in achieving a positive result or tending to produce a negative result