Your lupin may have been an annual, or it was loosely planted or the ground was too wet i.e. boggy conditions in winter. Lupins like a well-drained soil (including clay or sandy as long as it is well worked first but not chalk/lime)
They were covered in aphids - Lupin aphids must be sprayed at the first sign - usually April or May and again 7-10 days later. Any brand of aphid killer will see them off - systemic types are best such as Tumble bug, Roseclear, Polysect. Savona soap, a clove of garlic crushed and watered on is an organic option. Cut off flower spike, clear the leaf debris and allow to reflower if infested. A protective fungicide is advisable when the leaves first emerge in the spring.
Use bird friendly ferris iron based slugs pellets. You must keep putting them down too, especially after very wet weather. Also try liquid Slugclear or Sluggit watered onto the plants after a heavy shower. Put pellets down in the autumn too when slugs are laying their eggs to overwinter. Organic alternatives are copper rings, coffee granules, egg shells, human hair, grit, beer traps, copper filings and cooking bran.
No they didn't!! Lupins do not revert contrary to popular folklore. As the seed from the mother plant does not come true to colour, you must not let the flowers set seed and drop them around the original plant. If you do, the seeds will grow amongst the mother plant and seedlings will come up a different colour making it appear that the original has reverted especially if the original dies. Basal cuttings in February or splitting big plants in March is the only way to propagate your favourite lupin and ensure the same colour. They make a lovely cut flower if you plunge straight into water after picking to avoid an airlock forming in the stem.