Make sure you’re pitching enough yeast. You can find proper pitching rates on your yeast company’s website. If you are brewing a high gravity beer above 1.060-1.100 you can make a yeast starter to double or triple the basic yeast pitch. It’s helpful to make sure the yeast is within 5 degrees of your wort temperature prior to pitching. Large temperature discrepancies can shock the yeast and cause off flavors or poor fermentation.