Bin Temperature not holding

I have a 18 qt sterilite plastic bin set up for my baby ball python.. I have a heat strip underneath for the UTH heat as of right now, and i have a temperature probe under the substrate right on top of the heat source to gage the hotspot heat which is stays around 90 degrees and i have temperature and hydrometer gauges about 1 1/2 inches above the substrate.. I recently changed from a terrarium because it wasnt holding the humidity at all..

Right now the humidity is staying between 70-80 and the hotspot temp around 90. My problem is that the rest of the bin is sticking around 75 degrees right now.. Isnt it supposed to be around 80-85? and how can i fix this problem ? I have a daylight blue reptile bulb 40w from zoo med that i could put on a lamp that has a bendy neck to aim it towards the bin? or what would be the best way to get some heat in there? .

Thanks

I covered my rack with a blanket during night time. Daytime gets a little sun into the bins and heats them up a tad bit like it would in the wild

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