i think its caused by a combination of increased lag and increased population causing players to seem to not getting as many shells per dig, or may the shovels may have some bugs in lag where the shovel is counted as being used without actually digging, making it seem like its breaking faster.
its also most likely because jobs no longer seem to be as profitable, as while the amounts made stay the same, the prices of things goes up higher and higher so by comparison it seems the amount made is lower.
one thing thats for certain, players are experiencing something that makes it seem like the jobs arent making as much. no way players all just happen to be lying about the same thing for no reason.