Help with thumper use

[PossumPie]

I decided to use my thumper on a all-grain sourmash run. I had 11 gallons of mash so I had to divide it into 2 fermentors I fermented on the grain. ABV before ferment was 1.060 and after was 1.020...It stalled and nothing I did could drop it to my (hoped for) 1.010. I charged my 1.3 gallon oak cask thumper with 2qts of feints from a run back 3 months ago and fired everything up. When the stream started, I did an alcohol reading and got 40%. This shocked me b/c on stripping runs without thumper I generally get over 80%. I then remembered reading something about thumpers balancing out the product to a more even proof through the run. After an hour I was getting 20% and at about 3hrs I was getting 5% and shut off the still. Taste is sour with a small bit of grunge (wet cardboard of tails) but has good potential when spirit run is done.

Started with approx 4 gallons of wash in the still, calculator said approx 5% ABV, so I should have gotten about 4qts of low wines at 20% ABV. I only got 2.5qts at 20% ABV. Either something was wrong in my calculations or in my setup or I'm misunderstanding something. My second run is 6 gallons. I generally don't have to do a spirit run using a thumper but with the low ABV and having 6 more gallons to go, I will. I checked the ABV of the feints in the still when I dumped it and they were 0%. Don't know the ABV in the thumper, I'm not taking it apart.

EDIT:
Second run started at 40% ABV. If I didn't know better, I'd think there was a leak, but I checked all connections. I'll collect everything down to 5%, tear the thumper off, reconnect the condenser, and run a spirit run. I can't keg 20% ABV...