2x3 Gradient Question

[RC Maple]

I have a question about losing gradient. First off I have a 2x3 divided pan with a thermometer next to the draw off valve. I check with a hydrometer before pulling off. I always have a day or two when I boil that I can watch the thermometer slowly climb to just about syrup then fall off. Yesterday I boiled for 7-8 hours with a pan that had some sweet in it to start and never drew off once. I ran 50-55 gallons of sap that tested between 2 and 3%. I watched the thermometer climb to 218-219 twice but never quite got there. Most of the afternoon it went between 214 and 217. I have a preheater that sits on the back of the pan and slowly trickles into the first section trying to maintain depth at an inch. I seem to have noticed this more since I began using the preheater. Before I would watch the pan level and add sap as it got below and inch and a quarter. My thinking is that by adding more sap at a time I got more "push" and my sections maintained more gradient than by adding sap continually but more slowly. Looking at my pan last night it did all look the same color in each of the three sections. Most days I draw off regularly. It could just be the conditions for the day but I have had it before and never been able to put my finger on why it is happening. Could it be that on certain days the evaporation rate isn't great enough to be introducing sap fast enough to keep the gradient? It looked like my boil was pretty good all day though, not noticeably less than what I usually have. I'm not worried, all that syrup is still in the pan but it is rewarding to be able to draw some off.
Anyone else have this from time to time?

[maple creek]

Keep putting sap to it, you will eventually get syrup. As it adds sap to the back it keeps it diluted and the front will be the least diluted. I pull mine at 218-219 all day. Also run it low (about an 1")

[RC Maple]

I know it will eventually get to syrup I just can't get my head around the idea that it is almost there and then just falls off. I don't want to finish syrup in the house or anywhere else, if anything I want to thin to syrup not boil more but that's just me. I may make sure I have some nice thin hot burning pieces that I can add just as the thermometer is creeping up there and really see if I can get it up there that way. Most days it just goes right on up and I draw off from 220 down to 218 and usually end up heavy then adjust in the house. I went ahead and finished that batch on the turkey fryer and will jug it up tonight.