Some pumpkins are not accepted by farmers for trading. I think this is dependant on the orientation they were placed and possibly grew before mining/harvesting:

If pumpkins are selected from creative inventory they are accepted for trading.

If pumpkins are /given with no "data" value or a data value of 0 they are accepted for trading.

Any other data value I've tried when using /give results in the farmer not accepting the trade.

In survival and creative, pumpkins placed while facing North and subsequently mined/piston popped are accepted for trade, while pumpkins placed in any other direction before mining are not.

In survival some grown and harvested pumpkins (via mining or piston harvesting) are not accepted for trade, I've not been able to confirm if the growth direction affects the tradability.

The most recently obtained pumpkin seams to be the only one considered, so if for example you have 6 pumpkins which would be accepted for trading but then obtain a 7th that had previously be placed facing East/South/West, none will then be tradable. Similarly, if you have 11 pumpkins that are not accepted for trade, but then obtain a 12th that had previously been placed facing North, all 12 will be tradable. Placing pumpkins in chests etc between mining and attempting trade doesn't seem to change the outcome.