Plasma cutter troubles...

Been working on my Reverse Flow smoker, cutting .250" plates for the firebox. Replaced the tip/electrode cutting like a champ. Have the amperage turned all the way up. Hooked to my compressor, never drops below a regulated 100 psi. After making 20' or so of cuts it flat out struggles to blast thru. I am not moving any faster than I was earlier. Noticed when the struggle starts the tip has a small amount of slag built up on it. Replacement tips are a bit pricey, unless you use the overseas parts. Any suggestions?

Probably should add that the plate is new. Has been sitting on my trailer for a couple of months. Has some very light surface rust on it. I wire wheel where ever I am going to cut.

Thanks in advance for any and all help/advice....