That may turn out to be the answer, but I need to be clearer about the transmittance "direction" in your IR spectrum. If the transmittance is 100%, then the sample is not absorbing any IR radiation. All 100% of the IR light is passing through the sample without any being absorbed. If very little sample is present, the percent transmittances will be high, close to 100%, and more sample should be used.
Percent transmittances between around 11% and 53% are reasonable, but there are usually some regions of the spectrum where the percent transmittance is nearly 100%. I am visualizing a spectrum with 100% T at the top of the Y-axis of the graph and 0% T at the bottom (the "peaks" go downward). So in other words, if your percent transmittance is overall too low, the sample is absorbing too much radiation and less sample should be used.
Too much sample gives low percent transmittance (and high absorbance), while too little sample gives high percent transmittance (and low absorbance).
I hope the percent transmittance of this explanation is high! 
Steve
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