If I only had a dollar for every post here that claims … “I cleaned my plate thoroughly.” 
Before confirming a plate is clean, run a first layer test, as it will highlight any issues with the build surface.
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Simple layer test: Scale a cube primitive to 240x240x0.20mm, center it, and print. If the model isn’t a smooth sheet, the plate is likely contaminated.
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Use glue: Apply glue to see if it improves stiction properties.
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Painter’s tape: Apply tape to the build surface. If material sticks to the tape, it indicates contamination. This was the way 3D printing was done before heated beds.
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Check plate temperature: Use a non-contact thermometer (as low as $11) to verify even heating. Here’s a link. Uneven temperatures can indicate contact issues with the magnetic plate.
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Raise bed temp: Increase the bed temp by 20C or more and see if this affects the model stiction.
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Brims; The use of Brims can help with stiction.
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Deep clean: Avoid IPA for deep cleaning, most people use the drug store 75% IPA which itself may have contaminants. Instead, use Dawn dishwashing detergent and hot water with a nylon scrub pad. Details can be found in this link.
If you are using 75% drug store IPA, look on Amazon for 95% or better IPA or your local paint or hardware store may have denatured alcohol in the solvents section which is also a better cleaner albeit very expensive.
This is the most recent brand of IPA I purchased exclusively for cleaning. It was the cheapest I could find in bulk. But there are many others out there so buy on price not label.
In addition to that, get a high volume sprayer which will flood the surface of the plate. This is much more effective than using a rag and will wash away the contaminants with a forceful blast. Here’s two examples that I found effective. One is made of glass and the other, wide mouth. They both have a jet option which is useful and acts almost like a pressure washer.
This one has the highest blast pressure and is designed to be used with harsh chemicals. The only drawback is the narrow neck of the bottle.
