Why arent blue tits entering my box?

Thanks Hazy. Nice photo. I'm guessing/assuming 32mm is the vertical measurement, but the more crucial width is less?

Yes, I think the length was 32mm and the narrower width encouraged the blue tits to nest;   I liked the woodcrete boxes as we had woodpeckers but amazingly the wire mesh did protect the wooden boxes as long as the metal hole plate was added as didn't need to mesh the front with that in place.    We always had woodpeckers and they used to bring their young in;   we were so lucky and lost count of the species that we had visit our garden.... at least 52 including Crossbill juveniles who came to drink from the waterfall area of the pond, willow warbler, kingfisher and had so many unbelievable experiences with birds that came to trust us.   We had Jays, GSWoodpecker, Nuthatches, Wren, Long Tailed Tit that came within a metre from us as I guess we spent so much time working in the large garden they became unafraid of us, this confiding behaviour was primarily during their breeding season when food became the priority and they made a b-line for the mealworms.     The Great Tit would yo-yo up and down in front of the conservatory window to indicate that  the mealworm bowl was empty and then follow us to the garage where we replenished the dish and one day a wren was sounding off with chip...chip... chip round my feet whilst I was sitting on the patio having a coffee with a friend so I had to put down my cup and go get some more mealworms for it .   We never had house sparrows or starlings.   With birds, it's all about respect and trust and you can experience the most incredible encounters.    I understand you are not a fan of feeding year round and respect that and don't disagree with it but our experience with these close encounters were magical leaving a long lasting impression on us;  birds are incredibly clever and I spent a lot of time just watching various behaviours and traits which was so interesting especially the corvids.    I do miss that garden .... but not all the work involved !      Anyway, I deviate from the original post so will leave it there.

Certainly was an idyllic garden! Re feeding, yes, I'm not a fan. (lots of reasons as you know). However, I've also been trying to highlight over the years that people feed for different reasons, some of which are more valid than others. I've always argued that bringing birds closer (into gardens, within camera range etc....not particularly to hand feeding closeness!), is the most valid reason and understandable reason. Keeping individual birds fed in the depths of Winter, rather than leaving them to starve is also reasonably valid. I think a lot of people are mistaken though, when they think 24x7x365 is 'life or death', and we often have people asking about feeding or stopping feeding when on holidays for example. There is also very sketchy 'conservation evidence' for 24x7x365 garden feeding (though targeted supplementary feeding of endangered species, esp as part of reintroductions of cirl buntings for example is clearly valid). There is some evidence 24x7x365 is counter productive (as you know), and also there's the argument of disease spread being more of an issue with supplementary feeding. 

Bumped into my pal today who was doing a report about Willow Tits as part of BTO survey and it sounds very depressing with an ever declining population.    I didn't realise that their nesting areas are so specific and although we have plenty of suitable habitat on the parkland here with birch and other trees in the damp woodland there are none reported and it's looking dire generally for this dwindling species.   Also birds like blue tits can take over their nests.    Very sad if we lose this species.    There has been extensive improvements and woodland planting but maybe too late to save these birds.