Why can't I get agarose embedded HMW DNA to digest ?

[cavell at BBSRC.AC.UK]

I am trying to do some long range physical mapping of the Brassica genome.

I obtain high moleclar weight DNA by embedding protoplasts in low melting

point agarose. These are then lysed and washed extensively using PMSF.

I follow standard protocols for digestion with rare cutting REN's but 

there seems to be a problem with diffusion of enzyme into blocks as I 

get little or no digestion.

Any Ideas ??