Macro fails to Record String Results, Step Through Works

I have a table of timed data points for solar power generation. I want to be able to select a data range (ie all data in one month), generate a trendline, and record the the trendline formula in a table.

Sheet7 is a large table of data, with columns "P", and "W" of interest for now.
Sheet13 is where I am trying to record my table of trends for comparison.

To that end I made this macro. It
-makes a table of unique year/month combinations in sheet 13
-loops through each item in the table and generates a range object including the data for that month
-charts the data and extracts the trendline formula
-records the formula in the table in sheet13

The problem is the macro returns blank strings if it is allowed to run like normal. But if I step through in debug mode, the results are recorded as expected.

the problem seems the string assignments in "getTrend()"

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I got through my one year of data by hitting F8 between strategically placed code breaks, but this will be a major pain when I need to run multiple years of data. Any advice on fixing this would be greatly appreciated.


Here is my code in entirety
File with some example data is attached. All other columns were cleared to get it under 1000kB limit.

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I have solved my problem by finding out how to avoid the charts and strings entirely, using the LinEst Function

I replaced the "GetTrend" with the following.

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Why do I always do it the hard way first?

That said, I would still appreciate feedback on what was failing in my first macro, so that I can avoid making the same mistake again.
Regards Truk2