A grouping of pumpkins where everything is roughly the same importance gives the eye nowhere to land. It reads as a quantity of pumpkins rather than as an arrangement.
The fix is choosing one piece to lead and arranging everything else in support of it.
What makes a hero
Three qualities, and a piece only needs one.
Size. The most obvious claim. A jumbo statement pumpkin is a hero by default, which is most of why it's included from The Classic up.
Color. A piece that differs from everything around it takes the eye whether it's the largest or not. A single pale heirloom in a warm orange grouping does this; so does one deep green piece among neutrals.
Character. An extravagant curling stem, heavy ribbing, a warty specialty variety, an unusual shape. This is the most interesting kind of hero and the easiest to overlook, because it isn't the biggest thing in the box.
If you have a jumbo, that's your hero and the decision is made. If you don't, walk the delivery and pick the piece you keep looking at.
Where it goes
Low and central to the main grouping, where the eye enters the arrangement.
Not on top of a stack, which makes it look precarious and puts it above the natural sightline. Not at the back, where it gets crowded. Not on a step above eye level. The hero sits on the ground or on the straw bale, at the visual center of the mass, facing the street.
If your display has two groupings, the hero goes in the larger one. The second grouping gets your next-best piece, and it should be clearly second.
Give it room
The most common mistake is burying the hero in support pieces so that its outline disappears.
Leave a little air around it. Not a gap — the grouping still needs to read as one mass — but enough that the shape of the hero is legible rather than merged into the pumpkins beside it.
Height helps too. If the pieces around it are smaller and lower, its outline reads against the background instead of against them.
Orient it deliberately
Turn the hero so its best face points at the street, and if it has a stem worth seeing, make sure the stem is visible in that view.
Then check from the sidewalk rather than from the doorway. The angle you'll see it from every day is not the angle everyone else sees.
One hero, not three
Two large pumpkins of similar size compete, and competing focal points cancel each other out. If you have a second jumbo, put real distance between them — opposite sides of the entry, or one at the door and one at the bottom of the steps — so each leads its own grouping.
Three of equal weight in one arrangement is a pattern rather than a composition. Space them, subordinate two of them, or split them across the display.