Mahonia is a big shrub, so it is not ideal for a pot. It may be root bound or just hungry - you could try feeding it.
If you want insects, there are container plants which will attract them, but some ornamentals may be better suited to give a long season of nectar. Crocus are great for bees, as are violas. Hellebores and native primroses are other early flowers bees love. You could do a mixed woodland style planter that would look fairly natural.
Mid to late summer there are a lot of the daisy type flowers like rudbeckia or heleniums. If you mixed a grass in there, it could be a mini prairie style planter that wouldn't look too neat and contrived.
If you go for a couple of decent sized pots or troughs, rather than lots of small ones, you could pack a fair bit onto a patio