Offers may be hidden for one of three reasons:

You don't have enough balance, your current balance must match your maximum trade offer. Ei; your minimum is 100 and maximum is 1000 for an offer, your wallet balance must be atleast 1000.

You did not respond to a new trade within a reasonable timeframe, so your offer was automatically disabled to keep the marketplace healthy. When this happens, you will receive an email with more information. Your account may be restricted. If this is the case, you will need to contact the customer support team.

I opened a trade but the other user has not responded.
If a user has not responded, you can close the trade. After a short while, an offer by a non-responding user will be put on hold automatically and the user will need to manually restore it.
If you have opened a trade outside their standard operation hours then you will need to be patient and wait for a response.