I usually use ithis for drawing, but last week, I left it some days loaded with ink for a few days, and now it does not work properly. I usually use it for drawing, but last week, I left it loaded with ink for a few days, and now it does not work properly.

you need a comma before "and" if the two sentences that are being separated can be sentences by themselves

I usually use ithis for drawing, but last week I left it some days loaded with ink for a few days and now it does not work properly. I usually use it for drawing, but last week I left it loaded with ink for a few days and now it does not work properly.

I usually use ithis for drawing, but last week I left, it some days loaded with ink, and others not. and now it does not work properly. I usually use it for drawing, but last week I left, it some days loaded with ink, and others not. and now it does not work properly.

This implies it is right in your hand and you’re showing me; that would be the only appropriate context to refer to it physically. I added “and others not”; others refers to the other days you didn’t as you said “some days”; this is an incomplete clause and implies a comparison

I disassembled and cleaned it some days ago, but noit doesn't work fine yet. I disassembled and cleaned it some days ago, but it doesn't work fine yet.

I disassembled and cleaned it some days ago, but it still does not work fine yetwell. I disassembled and cleaned it some days ago, but it still does not work well.

I disassembled and cleaned it some days ago, but not work fine yetyet it fails me. I disassembled and cleaned it some days ago, yet it fails me.

More nature and dramatic

Of course, this is not a bighuge problem, but I will need my fountain pen in the nextcoming weeks, and I would like that it wasit to be ok. Of course, this is not a huge problem, but I will need my fountain pen in the coming weeks, and I would like it to be ok.

"huge problem" is more natural

Of course, this is not a big problem, but I will need my fountain pen in the next few weeks and I would like that it was oko work right. Of course, this is not a big problem, but I will need my fountain pen in the next few weeks and I would like to work right.

Of course, this is not a big problem, but I will probably need my fountain pen in the next few weeks and I would like that it was ok.thus I need it to be functional. Of course, this is not a big problem, but I will probably need my fountain pen in the next few weeks and thus I need it to be functional.

Style