Please provide a more specific description of what you mean by "this field disappears."
Do you mean the content you entered disappears after you tab or click out of the portal?
The portal record disappears?
The field become unenterable?
Is your portal sorted or filtered?
What are the key/match fields for the portal relationship, and are any of these defined to change based on other entries? (Auto-enter calcs, lookups, etc?)
You indicated that you reviewed some of these, but not what you found. Need some more real-world details to do troubleshooting.
Sorry for not being specific enough.
When I talk about dissapearig fields I mean that the text box hides itself when I'm writing on it. The data does not dissapear because when the focus goes out the text box, the data entered appears again. There is no lost of information, just it makes invisible (It only happens with a text box in a row, not the entire row). You can alter the field, delete the data or re-enter it again. The only problem is that the text box hides when the focus gets in.
The portal is not sorted or filtered, and there is only a key(an id) to another table.There are no change based entries. I think it's more a Format issue than a "Programming" or Design issue, but I'm not able to find the reason. When I reviewed it, and couldn't see anything strange...
You don't have the upper or lower bounds of the field box overlapping the bounds of the portal by any chance? (Or even contingent).
It does sound like a formatting issue.
In addition to the positioning of the field completely within the portal top bottom, I suggest the top of the field be 1 pixel below the top edge of the portal, and the portal row be at least 1 pixel below the lower edge of the field. You may need to increase the portal height to let the field be large enough for the font issues (next).
Then check your font family and font-size, keeping in mind that field heights must be higher than the font size to display correctly, often by several pixels, and by usually even more on Windows than on Macs.
Color choices can also cause things to appear and disappear, depending on the field background color, the layout background color if the field is transparent, and the font color. These tend to change a little when clicking into and out of fields, depending on OS system color choices. Check you system color choices as well if this happening on a single computer but is OK on others.