regression question!

I am not the best at stats, but this is what I am trying to do:

I am performing regression between a DV and IV1. Now I have another variable IV2, and I am trying to gauge the moderation effect of IV2 on IV1-DV association. So when I am performing lm(DV ~ IV1*IV2), the p-value for IV1:IV2 interaction is 0.8. Next, I split my IV1 and DV data into two groups depending on IV2 values (at median) - "IV2-low" group and "IV2-high" group. Now if I run lm(DV ~ IV1) in both groups independently, I find the regression p-value 0.003 in IV2-high group, but p-value of 0.6 in IV2-low group. Which essentially means that the association between DV and IV1 is different between IV2-low and high groups, but I don't see any effect of IV1:IV2 interaction.

I am not sure how to handle this situation now. Our initial result was generated by splitting the datasets my IV2 median and everything was validated experimentally to show that the DV-IV1 association indeed depends on IV2; but the reviewer suggested to run a single regression (not splitting the dataset) in which we don't see any effect!

Please suggest why is this happening and how to go about the issue!