Measure the results of the samples at the same temperature as the standard. These are some of the easiest errors to overlook. Other problems can be due to product occlusion. See the following question on occlusion to find all sources of occlusion. Finally the other area to look into is the sampling point.
Occlusion occurs when all of the drug product that is available has been collected.
The filter may be absorbing drug substance, also the cannulas, syringes, vessels, and shafts can also be guilty of holding on to your drug product. Some may still reside in a non-disintegrating tablet, or undeaggregated material. There is also some variability with content uniformity that must be taken into account. I recently got a call from headquarters. They had just this happen at the research lab.