Description
Two values before and two values after the null to be filled are required to calculate the temporal trend.
Solution
Inspect the timestamps of your missing values.
When using Temporal Trend to fill values, the null value being filled must have at least two time periods with values before it and at least two time periods with values after it in order to be filled. Because of this requirement, nulls existing in the first two or last two time steps will always be unable to fill using Temporal Trend.