The compost in the pots may be drying out to quick for the plants you mentioned. You could try geraniums(pelargonium) as they are more tolerant. See you next season from Borobabs.

What size are the pots (height and width)?

Thank you for the advice Borobabs, look out for an old lady with a Leicester scarf!
Thank you Bamboo, they are 36 x 36 x 14.5

mm, cm or feet? I'm assuming it can't be feet... but even cm seems large for stacking pots.... usually they have 3 or 5 different sizes

WE ARE THE CHAMPIONS MY FRIEND ?

WOW JUST WOW

Sorry I forgot the question?

Are they the sort that are basically round but divided into three, and you stack them at an angle to the one below? If these are what you have you'll know what I'm trying to say. I've tried growing strawberries on a three tiered on but the success was only mediocre. You could ry stacking them in two lots, a three and a two. Perhaps you could try fibrous rooted begonias, especially if their position will be in partial shade, as they are pretty tolerant.