I've just had the same problem, and it was ink-jet paper. Not sure if the problem is the printer or the paper. I was advised to try a coat of matte spray on it first, but that made the colours wash out. I'm using an HP 4500 OfficeJet, with genuine HP ink. Tried various print settings other than the recommended ones, including maximum DPI and 'best' quality, no joy. In fact it seemed to make it worse, the yellow I wanted came out rather orange.
It was my first time printing decals in colour, would love to find out how to do it better.