Kitkat, this has been covered very thoroughly in an earlier thread. Do not tip the dregs from a few longnecks in and expect your wort to fire up. Just because there are a teaspoonful of yeast sediment in each, does not mean all these yeast are alive. They are probably 99% dead.
Mutant yeasts and bacteria are much faster at multiplying and will quickly take over in numbers from your wanted yeast. Your beer will be infected.
Big rule of brewing, pitch enough happy active yeast into your beer to get the beer fermenting.
Make a starter, and only brew with it when you are happy about its activity. Keep a pack or two of dry just in case.