>> My soil is a mixture of potting soil, cow manure and sand all mixed together in fairly equal amounts.
>> and filled each with about 2 inches of sand before adding my planting medium mentioned before.
Sometimes a sudden change in he texture of the planting medium breaks the wicking or "capillary connectedness" of the soil column. If so, a lot of water might be accumulating right at the boundary between your potting mix and the plain sand. If too much accumulates instead of draining, anaerobic soil and root rot follow quickly.
The question was a very good one: "does water come flooding out the bottom?" If a very dry pot gains a lot of weight after a heavy watering, you might not have good ENOUGH drainage. None of your three ingredient are necessarily coarse, like 1/10 inch grains or 1/8 inch grains.
Manure, even composted, is very fine and water-retaining. Most non-pro commercial potting mixes have a lot of fine peat, which is killer fine and killer water-retaining, and I don't mean "killer" in a GOOD way!
If that had been 1/3 very light and open, airy potting mix" plus 1/3 medium grit or crushed stone around 1/8", or very coarse Perlite great!
My belief is that tall 5 gallon buckets need even airier mix than smaller containers.
