LED Lights - certain spot not working

[Boomhauer]

I bought some new LED lights to get ready for a new tree we'll add next Christmas. Each strand has 70 bulbs. I bought strands in red, white, and blue, with the intent of swapping bulbs so each strand was RWB in order. 

The first grouping of RWB went fine, and all bulbs work on all 3 strands. On the second attempt, one particular spot is unlit, and it's the same position on each strand. From the plug, it's bulb 36 out of 70, so right at the halfway point. I've tried replacing the bulbs, but that spot won't light. All other bulbs on each strand are illuminated. I tried replacing the fuses, but no change. 

Also, in removing and replacing the bulbs, certain bulbs seemed bigger and wouldn't fit. They all look to be the same size, but a handful of times I swapped out a spare bulb because a particular bulb was extremely tight and didn't want to fit in a spot. Maybe certain positions have openings a tiny bit bigger for bigger bulb plugs? I don't know.

Has anyone else encountered such a thing? Is there a way to fix it? I feel it has to be something specific because the same spot is bad on each strand. Also, I don't want to start swapping bulbs on additional strands only to have the same issue come up. 

Thanks for your advice.

[Rich in Las Vegas]

If this is set up the same way many of the light strings I have, the design is to have two series circuits with 35 bulbs in each series.  If you follow the way the wires come out of each base, there will be four bases with 3 wires coming out the bottom and the rest with just two.  There is one at each end and two adjacent to each other at the center of the string.Those four (three wire base) lights will have larger bases than the rest and the insert holding the bulb is also larger.  It is just  a way to feed the second part of the string without having to run an extra wire along the entire string.  If you try to put a two wire bulb base in a three wire base it will be too loose and the other way around will never go into the space,