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Sound like you didn't harden completely. Could be caused by a couple of things, either not getting the steel hot enough to start with or not cooling it fast enough. You didn't mention what type of steel you're using, so I cant give you certain advise, but the best thing you can do is find a data sheet for your steel and stick to it religiously. For simple carbon steels, O1, 1080, 1095 and the like, you need to get them just a bit past the curie point, about 1500f, then quench. Keep a magnet next to whatever heat source you're using, once the blade doesn't stick to it it's hot enough. Don't go by color! Color is nearly useless, doubly so when you consider that the color a blade glows will change depending on the ambient light.
What quenchant you're using is almost more important than the temperature. Don't fall for the internet crap, a bucket of used motor oil is not what you want to be using. For most oil quenching steels, which is what most plain carbon steel are, a few gallons of warm canola or peanut oil will work beautifully. However, some steels require a faster quench, steels like 1095, the w series steel and the like. Quenching a 1095 blade into canola oil, especially if it cold/cool, will result in exactly what you're describing; a blade that's been sorta hardened but not really. For these steels, you need something that'll cool the blade faster. For most people, that means water or brine, which also means a lot of cracks. If you plan on working with a lot of 1095 or the like, I recommend investing in a bucket of Parks 50. It's expensive at $150 a bucket, but I've never lost a blade in it.
Long story short, make sure you've got your austinization temperatures right and make sure to match your quench medium to the steel
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