Dell monitor does not run on native resolution of 1600x900 pixel when connected to early macbook pro retina 2015 via thunderbolt
There is nothing wrong with your monitors, neither your Mac. That's a compatibility issue with your adapter. There are many comments on Amazon complaining about the same issue on similar converters. That seems, unfortunately, a hardware fault.

Example: http://www.amazon.com/gp/aw/cr/B00NH13I08/ref=mw_dp_cr