Re: Is this the effect of missing steps on a delta?

For deltas you should pass the paper test at 4 points. Positions close to the three towers and the middle. It could be that you have the middle point lower than the outer bed positions (concave surface), that why you can print the outer perimeters ok, but not the infill.
Normally, you correct this by adjusting delta_radius in configuration. You can change the delta radius indirectly by changing the smooth_rod_offset (or any other of the delta parameters) to lower or raise the nozzle at the center. If I recall correctly, in your case you should decrease the smooth_rod_offset a bit and see if it improves things. But really, check if you can pass the paper test at all 4 points, if you cannot, you have a problem with calibration. If you can, you need to take another look at z-height and extruder calibration.

In case you have autolevel probe installed, you can also use the Marlin firmware from Rich Catell which will do all of the calibration for you (its on github, google it). Richs Marlin can adjust most of the parameters by itself, the endstops, the delta_radius and the diagonal rod length as well, and a couple of few other things not in the original firmware.