Lemon trees shrivelling

[Sweyn]

Not sure if this should be here or on the fruit forum, but I have a problem with my small Lemon trees. They are 2 years and only 2 out of 4 of them are shrivelling.

One is quite small and has shrivelled a lot. Its main branch has tipped over and it now looks dead.

The other is about 1 metre tall and is shrivelling less. The leaves are hanging down, but only some are shrivelled. This is about 2 months since I started using citrus tree fertiliser on them.

Any ideas what could be causing this?