Depend on the atmosphere you perform the synthesis and also the amounts of the monomer you add. I presume you are doing this in a solvent. make sure you clean up the solvent before you run the reaction. 

Adding extra monomer is a trick used to increase the hardness of the polymer as the excess reacts to form a polyurea over time, it increases stiffness and is irreversible. However, there is another trick, which is to use a high molecular weight thermoplastic as a sort of non reactive fluidiser which plasticises the matrix only when heated. So your glue is hard during normal temps but fluid when heated. You can then cast and recast your material to your hearts desire but it will still be hard when cold and runny when hot.

For systems based upon polyols, following factors will affect your TPU properties (if not controlled):

Molecular weight (Mw) and MWD of polyols (lot-to-lot variability is common), isomer ratio of diisocyanate (2,4..2,6..etc.), chain extender purity, %NCO, moisture (moisture will react with %NCO to form CO2 gas + amine). And if that happens your %NCO drops leading to a lower hardness and lower modulus, Tg etc.  So, most critical factors includes: keep moisture away, purity of monomers, MW and MWD of polyol including prepolymer (PP), %NCO (constant stoichiometry). So, I would suggest make sure you are not only polymerizing in dry atmosphere but also all your monomers are moisture free. Should always do Karl Fisher on PP to make sure you have constant %NCO. If that’s the case and you have added exact amounts of polyol/chain extender then you can ensure constant stoichiometry and hence consistent TPU properties. Hope this helps.

Did you check the MDI for dimer (uretdione) content, is it a clear melt at 46°C? If not, either filter or decant it or distill it. Further, did you check the polycarbonate diol for water (it has to be less than 0.02 %) and traces of decomposition due to added amine catalyst? By using DMF with only very low amine content (< 1 ppm) the amine will damage the chains of the polycarbonate, the same is with tertiary amine catalysts. Is the time of preparation of all the batches always identical? With polycarbonates, minute changes may have a great effect on your PUR properties.

Thank you so much for answering my question. All advices do help me a lot for improving my work. 

I thought, synthesizing TPU wasn't that hard. But, now I know, synthesizing TPU isn't that easy. There are many factors to control. 

Did you ckeck your DMF for free amines? Traces of free amines will affect the structure (molecular weight, chain length distribution etc.) of your polycarbonate diol. Be sure that there are as well neither water in it or dimers in the MDI. Use freshly disstilled components. The index should be somewhat higher than 100, i. e. 102 to 104.