Hi ya and welcome
I also use an insulin pump..
There's a couple of things that you could consider if it's just on odd occasions..
First one is Bubbles going through the tubing..
Even though we do constantly check for bubbles, but because of how insulin adsorbs and releases air with fluctuations in temperature there's always a possibility of air being released from the insulin and going through the tubing without us noticing until our BG's start rising for no apparent reason.
To prevent this from happening, I always wear my pump with the connector facing downwards, then if any air bubbles do develop with temperature changes the bubble raises to the end of the cartridge well away from the tubing,
Another thing again concerning Air Bubbles, sometimes air get sucked in through the connector because it's either wasn't tightened sufficiently or it's worked slightly lose... It's wise to regularly check that they connector is securely tightened.
Another area to consider!
When you get these unexpected highs, when was the last time you changed your infusion set, and does it seem to happen within the same area!
As it could be problems with adsorption either caused by leaving in the cannula too long, and/or that particular area if used you need to change the infusion set earlier to elsewhere. I have this on my left side, if I use my left side I have to change this at 2 1/2 days other wise my levels will start to creep up, on my right side I can actually leave my infusion set in for longer than 3 days without any adsorption problems at all... The latter isn't wise though as could eventually cause adsorption problems over time.
As to why your levels don't go down very quickly, is probably based around insulin resistance the higher the blood glucose level the more resistance it becomes to insulin. Might be that you've require an high ratio of insulin for correction, but also may need to increase your basal rate for a while to bring it back into range
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