There are a few occasions when the Premetz model is still unable to achieve a good fit whether using the basic model, or the more complex analysis. There are generally found to be 2 distinct causes for this behaviour.
The most common is that the flow rate through the filter slows virtually to a stop (effectively the filter is plugged or blocked), or there is insufficient metal used in the test. In these cases, the weight vs time plot shows a distinct plateau as shown.
In this case the algorithm searches for a good fit by disposing of later data points: in this example it only uses the first 80 seconds worth of data to find a solution (the data is said to be truncated).