A few things you can consider checking that I have noticed over the years to check when using meter summation. These are the most common reasons I have seen that can cause loads to fail to get allocated.
1) Is the model, the Thiessen polygons, and the meter data all in the same spatial projection? Any type of spatial join is very dependent on using the same spatial projection. Two data sources can look visually like they overlap when different data sets are reprojected automatically (like what ArcGIS does) , but will not "overlap" in a spatial projection because it assumes the coordinates of all the data sets are identical and ignores the spatial projection which will adjust the origin for different projections. Check each closely, as sometimes there are projections that look very similar but may have slight differences, especially if using state plane coordinates. Different projections would change how the software perceives the location of the data and can impact allocation.
2) Are all of the meters within the Thiessen polygon boundaries? Any meters outside the Thiessen polygons would not get counted as the software sums up all meters inside the Thiessen polygons.
3) Are any meters getting assigned to inactive elements?If so the allocation report would show the total allocated as correct, but the input table (if only showing active elements) would show less load "apparently" allocated. Check your Thiessens are all based on active elements and recreate them when necessary so allocation is applied only to active elements.
Often one of these three issues has often the cause for unallocated meter loadings in many allocations I have troubleshot over the years and may be worthwhile checking.
Patrick