Why does adding more coils make my electromagnet weaker?
Yes, it’s increased resistance. Why? With a few simplifications, the principles are simple to understand. 


The generated magnetic field is proportional to the current through the coils and to the number of coils (per unit length). B ~ n*I
The resistance of the electromagnet is proportional to the number of coils and to radius of the coils (technically to the length, but that’s just a 2pi constant). R ~ n*r
Being at steady state, the current through the electromagnet is inversely proportional to the total resistance. I ~ 1/R


So, if all of the turns are the same radius (which would be physically impossible) you would have: B ~ n/(n*r) ~ 1/r regardless of the number of turns. 

But as the radius of the turns increase as you add more layers of wire, in reality you have: B ~ n/(n1*r1+n2*r2+n3*r3...) where r1 &lt; r2 &lt; r3... and n=n1+n2+n3... Which decreases as you add more layers because of the increasing radius. 

The optimum number of turns will be achieved when the increase in coil resistance  is exactly balanced by the increased voltage provided due to the reduction of load to the battery.