"Hole" in the middle of Southern bands?

[Randy Hardy]

 Here's a weird problem: recently we have started seeing empty centers to
intense bands in our PCR-Southerns (amplify, blot, probe with internal
fragment, autorad).  Previous months of blotting never showed this.  Now
past 2 weeks, several blots have it.  Sometimes only in certain lanes of a
single blot, sometimes all.  Sometimes appears as a clear area in the lane
(2 sides of band are hot), other times it's a "doughnut".  Anyone else
ever seen this and/or opinions as to what's causing it.  BTW, We're using
a StrataGene pressure blotter (have been for past year).  Thanks in
advance...