Taste panel testing and contest results suggest that it is possible to make very good beer using malt extracts, but many of us have at some time had problem extract batches. The degree of variation this study identifies suggests that we may not always have been at fault. Following are some guidelines for using malt extracts:
If possible, make at least a portion of your wort from malt, performing a "partial mash." In addition to enhancing flavor, it will add utilizable FAN.
If you can't at least partial-mash, try adding small quantities of a commercial yeast nutrient. Although it may not provide the full spectrum of amino acids necessary to avoid all fermentation problems, it may at least boost the attenuation rate.
Don't add sugar to an extract wort. It may already have all that it can stand.
Some yeasts, especially lager strains, are very sensitive to wort composition. Experiment with yeast-extract combinations.
Whenever possible, request analysis data from suppliers or manufacturers. They certainly generate such data as part of their quality assurance process, and knowing the extract's composition will help you to decide how to use it. An extract's FAN content may be perfectly adequate for a high-gravity beer but deficient for a lighter one.
Experience is the best teacher with each product you use. If it works, keep using it; if it doesn't, switch.