Hopes were high at Sixfields this season after a ninth-place finish last term but the sale of goalkeeper Mark Bunn to Blackburn and defender Gabor Gyepes to Cardiff has led to disruption in the back four.
Jackman said: "It's frustrating because we've lost certain players to bigger clubs and we haven't been able to bring anyone in on a permanent basis.
"We're reliant on loan players and that isn't ideal but it's the way the club has to function.
"We have to deal with that in the best way we can but it's down to the players that are out there. Losing those players is a reason but it can't be the sole reason.
"The players out there are good enough and it's hard to say why it has gone the way it has. It's such a fine line. They were all good players and that's the reason why they've all moved up the leagues."