DNA Precipitating in Sequencing Gel

[Dr M Fahmy]

My colleague has a problem that no-one here can solve.
After doing Ligation-mediated PCR on a genomic DNA sample, the dried pellet 
is resuspended in sequencing stop buffer (95% formamide, 20 mM EDTA, + dyes),
denatured, chilled and loaded onto a standard 6% urea/TBE sequencing gel.
Although this was successful a while ago, the sample when loaded now
solidifies on contact with the buffer, at the top of the gel. This
 has been repeated many times. All reagents have been changed apart from
some of the primers, the original genomic DNA sample and the tRNA for 
precipitation. Sample remains OK if no PCR carried out.
Does anyone have any suggestions as to what is happening or how to 
correct it?
Many thanks,

Magdy Fahmy