What's eating my plants

 

Can anyone help with any suggestions as to what is eating my plants.  A while ago I planted two types of lettuce, rocket and radish from seed under cloches.  The radishes have come up fine but one row of lettuce and the rocket only have plants at each end and not in the middle.  At first I thought it must have been the way I planted the seeds but yesterday I put in some young lettuce plants that I had grown under glass to fill the gaps and two in the middle have already been eaten.  I treated with slug nematodes before planting (which should still be effective) and I have some cabbage etc nearby which hasn't been touched.  Just out of interest the other row off lettuce was eaten completely except for a couple of plants each end.  Any suggestions as what could be doing this, whatever it is it seems to be sticking to a certain patch.

Many thanks

Sue

Slug nematodes are only effective against the underground slugs, not the surface ones - so I'd consider them as the first likely offenders.

Are the plants netted - pigeons or rabbits are an alternative.

Are the plants being pulled out, or are they just razed off at ground level?