Obviously if you have a spare a frame(s) of brood from a strong hive to give to the weak hive, it really helps to get them going; along with what the others have said. The more numbers of bees you have in a box, the faster they build up. One deep frame has over 6000 cells. Not all are filled with baby bees, but still adding a deep frame of brood to a weak hive greatly boosts their numbers (as the brood hatches out).
As Michael Palmer was preaching in his nuc presentation, if you have some spare nucs in the spring, you can use frames from them as “bee bombs” (his name) to boost weak production colonies in the spring.