Do you remember what amperage you used for that size rod? We had some 1/4" E7018 here but I just scrapped it before our AISC audit. The lincoln rep gave me a sample of them to try when they quit making the larger size E7028s.
We have run 5/32" E7018 here in our shop uphill, but not the 1/4".
edit: BTW, that shop should have provided you with a WPS(welding procedure) before you attemped a test. The WPS will have any special instructions detailed out and what amperage you have to set the machine at to weld that joint.
I give all of our applicants a WPS and plenty of scrap to get set up on, or practice, then lay down all the ground rules and what all I will be looking for. I advise them to leave the grinder on the bench and if they get in trouble and need to use it, to come get me and we'll discuss whether to grind or start over. I usually watch them place the root and a few hot passes then get out of their way and let them weld it out, maybe check in every now and again to see how it's going and about when to expect them to start capping it off.