incorrect preheat. Usually the problem is too low preheat but your description of slag building up leads me to think maybe you have too much preheat causing the steal to burn prematurely. Look at the edges of the cut section, does the torch side edge appear soft and melted? This would indicate too much preheat usually caused by an excess of acetylene.
The cross section of milled shapes varies over the width so the preheat that was adequate near the edge of the shape may not be enough near the middle. This problem can be compounded compounded by a heavy oxide coat which can deflect heat, too rapid a travel speed, improper torch orientation, or improper torch to work distance.
Can't say that I've ever noticed a difference between plate and structural steel so I'm confident some changes in your technique can solve the problem. Preheat the whole of the section not just the flange. Pay particular attention to the fillet where the web and flange meet as the steel is thickest here. Make sure the cutting tip is oriented such that one of the preheat nozzles is leading the cut and not offset. This seem like a small thing but was showed to me by an old hand and I notice the difference. Reduce your travel speed slightly. The best way to gauge your speed is to watch the stream of metal coming off the backside and slow down if it starts to deflect. You will have to travel faster near the edges of the section and slower near the middle. If these fail to cure your problem change to the next tip size up to increase preheat.
starbolin