Would help alot to know what type/model of DeLonghi it is. Is it a pump model with a water reservoir? A steam machine with a closed boiler system?
Vinegar is barely OK in a pinch, but I highly suggest using at least something like BrewRite, which is widely available at most grocery/department stores. Better yet would be food grade citric acid.
Have you tried cleaning the filter basket the grounds go in? Hold it up to light and you should be able to see through all the holes. If not it is clogged and this can be an issue. A good soak in something like OxyClean can make a big difference. It doesn't take much coffee/oils to clog these tiny holes. Later!