Should ipnat's statistics really be monotonically increasing?

# ipnat -s
mapped  in      17877109        out     15501105
added   442065  expired 0
no memory       14499   bad nat 19
inuse   2491
rules   44
wilds   4294967294

There comes a point where it seems one can't make new connections (as in
you have to be lucky, or try often). The ipf side of things is fine..
The "no memory" part above looks worrying - what type of memory is ipnat
running out of? What can one do about it?

Cheers,

Patrick