Hello. Is it a dwarf hamster that you have? One of the issues may be that the cage is too small for a hamster - it looks like the Savic Spelos or Rosewood Pico? Unfortunately people get sold cages that are far too small for a hamster to have normal behaviours and it sounds like he is hiding away a lot. I assume you'll need to clean it out regularly as well with it not being very big, which will also stress him and make him want to hide away more.
It is a total pain when you've bought a cage and need to replace it - same thing happened to me with our first hamster.
About the minimum size cage you'd want for dwarf hamster is the Ferplast duna multy, linked below - which is a perspex tank style cage. There is more floor space and you can add a house and cardboard tubes and hidey places - so the hamster doesn't have to disappear under the substrate to have places to hide.
Another popular barred cage for a dwarf hamster is the Hamster Heaven (with the penthouse and tubes removed as not suitable). That has a large front opening door which can make it easier with interaction and taming with your hamster.
https://www.portonaquapet.co.uk/shop...a-Multy-M36261
https://www.zooplus.co.uk/shop/small...r_cages/197362
If it's a syrian hamster you have then you'd be looking at slightly larger cages or the Hamster Heaven as a minimum size really.
The basics they need to have normal behaviours are
1) A good sized house that is dark inside (big enough to build a good cosy nest in) - so they have somewhere dark to retreat to. When they have a house that's dark inside they feel more confident aboiut coming out into the open. Ideally a house that is open underneath and sat directly on top of the substrate - so they can bury hoards under their nest.
2) A shelf or platform - something to sit under to feel safe, and something to climb up onto for somewhere to go - a good place to put a food bowl etc.
3) At least 3 to 4" of substrate.
4) A wheel that is the right size for the species. Anything smaller than 6" diameter is too small for any species of hamster - they can get back problems if they run in a wheel that's too small. For a dwarf hamster, 6 to 8" diameter is fine and for a Syrian 10 to 12" diameter - with a minimum of 8".
5) Enrichment - this is the key word these days - it isn't just the size of the cage but the amount of variety in it - lots of places to go and things to do - so tubes and floor tunnels, hidey places, little bridges to run over and hide under - that kind of thing.
The hamster then feels a bit more confident and happy and does more out and about in the cage. Having a house with a flat roof and a lift-off roof is a good idea too - it means you can check on the nest inside the house, without having to remove the house and the nest falling apart.
IT's also best not to do big weekly clean outs as that really stresses them. Spot cleaning is best - ie taking out a handful of soiled substrate and replacing it with a clean handful and mixing it in a bit so it smells familiar. With enough substrate in a larger cage you can just spot clean for 6 to 8 weeks easily. And even then it's best to only do "partial" cleans - ie don't clean everything at the same time. So when you do change the substrate, try and put a little bit of the old back on top (that is clean and dry) so it smells familiar. And clean the wheel a different week and any toys a different week again.
They get very fussy about their nest and hoard and it is best not to remove those unless they've been pee'd on - and then put new food back in exactly the same place as the old hoard. Dry hoards can be left for quite some time - and nests can be left alone as long as they're not pee'd in. Hamsters tend to refurbish their nests themselves.
With the current cage he probably feels quite exposed when he comes up from under the substrate, so hides underneath it.
With a larger cage with a house in it, and some overhead cover from a shelf, a house and the odd thing hanging from the roof, he will feel more secure coming out.
They also get scared by a hand coming from the top sometimes, with top entrances.