MMM Calibration

While training a model I encountered an issue during model calibration. We initially built a model using 2 years of data and then added synthetic spend data for a media channel over the recent 4 weeks. This channel had 0 contribution earlier, and after adding significant spend data for the last 4 weeks, it still showed 0 contribution.
To address this, we calibrated the model using incremental revenue and spends for the same 4-week period, but the channel continued to show 0 contribution. However, when we increased the training size, we started to see contribution for this media channel.
Could anyone share best practices or insights on how to effectively calibrate the model for recent periods?

@gufengzhou @laresbernardo