If your espresso tastes sour, chances are it wasn't percolated at a high enough temperature. Coffee beans release different compounds at different points in the extraction process. For drip coffee, a sour note often means the coffee was percolated too quickly. For espresso, however, there's generally a set, much faster brewing rate. Therefore, if only the first, more acidic notes were well extracted, then either the water wasn't hot enough or the coffee grounds were too coarse for espresso's quick, pressurized brewing.