Solderability problem with tin plating

We know that tin plating gives excellent solderability but recently we have encountered difficult solderability at our tin plating (iron base substrate) electrical terminal. I would like to know what has caused the difficult solderability of the tin plating? I do not reckon is the formation of oxide layer as our SEM/EDX result does not detect the presence of oxygen.

Thank you.

It would be handy to see the parts concerned, but if this is not possible, so be it. I am surprised there is no evidence of oxygen on the tin because tin easily forms an oxide. What other elements have been detected? I wonder if insufficient tin has been put down and when you have heated the tinned substrate as you solder, the tin has diffused into the iron base to form iron-tin alloys. This could easily cause problems with solder as the iron-tin alloy does not wet as easily as iron.