The similar thread between the four commonly accepted Maiden misfires - NPftD, FotD, TXF and VXI - are, in my opinion:
1) Especially true to the ones with Bruce, everyone sounds a little off. Bruce seems like he's struggling a bit. The guitars are still playing in unison like only guitars in Maiden can, but none of it is nearly as interesting or powerful as, for example, the cool stuff they did in "Hallowed Be Thy Name" or "Revelations".
2) I think the music's lacking in the intangibles. It sounds formulaic, passionless and stale. After seven albums of not straying far from their comfort zone, I think they finally got to the point where they could churn out riffs and vocals easily, but there was no excitement about anything they were doing.
3) A summation of the above points, overall, they just weren't as hooky as older Maiden albums. You can still groove and rock to the songs, but you don't remember them like you did "Powerslave" or "Wasted Years".