How high gravity are you going for? As long as primary fermentaion was moving along, it isn't necessary. A hydrometer is a big help. If you do something like a barleywine at 10% abv, the OG is going to start somewhere at 1.100. The final gravity will only get down to maybe 1.025. If it stalls somewhere around 1.035-1.040 or even higher, more yeast can be help. Make a good yeast starter, or even better, pitch on top of the entire yeast from a previous batch and you'll most likely avoid this problem.