Hmm.. I had a few and they are about 20" tall now.

Prone to slugs.

Where are they? slug biomass. Slugs eat them the moment they emerge from the ground in spring. I treat them as annuals. I also start putting out slug bait as soon as the early spring bulbs are up, that helps somewhat. I might get one or two survive that way.

Do you remember what type of delphiniums you planted? I have spent thousands of dollars on delphiniums over the past decade; there are only 2 left in my garden. I truly believe it is the location, which gets soaked regularly because it's a rose bed. Everywhere else in my yard, the delphiniums are an annual, even the native ones. The 2 surviving delphiniums I have are 1 Pacific Giant(blue colored) and Black Knight.

And I also second the slug bait, also make sure your husband doesn't go digging where he shouldn't be digging; I have lost one or two in this way LOL!

Old dahlia person told me that she truly believes the slugs are in the soil and begin eating before plants emerge. I believed her and have grown great dahlias since I dosed them in a ring around and covered with a little top soil. Then I gave them another shot as soon as I noticed them coming up. Seems to work. Maybe try it with our delphiniums. Good Luck!