doing a loop repeatedly only for an specific value

Hi,

I have 4 Columns which have these values:

Column A : a, b, c, d, e
Column B : 100, 200, 300, 400, 500
Column C : 150, 250, 350, 450, 550
Column D : (Get random numbers by macro)

And this a macro which generate random numbers for Column D and check if generated random number there is between ranges of Column B and C then macro returns correspond value from Column A and write it in Column F and if the generated number there is not into any range, it returns "NA" string value .

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Now I need a double check for those cells in Column F which have "NA" value and generate new random numbers only for those cells and do comparison till all cells get a value in range and finally no cell will be remained with "NA" value.
How can I do double check till get results in range for all cells in Column F?