What should I do when my giveaway's winners haven't sent feedback in several days?

if they don't accept a win within a week of you clicking send you can open a reroll ticket(has to be a week+ after clicking send, so any delay on your part needs to be accounted for to). If you send a gift through a different method than the key system on SG you will need to provide proof of it being sent with timestamps.

to note you won't get any CV for RPG maker as it was free in the past and is now 0CV

Taken from the guidelines:

Winners will need to correctly mark their gift as received or not received within one week of the gift being marked as sent by the giveaway creator. It is also necessary to keep this feedback up-to-date if the status of the gift changes.

Please be patient and allow one week for people to receive their game. If after one week they do not respond, I usually send them a reminder on an old giveaway or message on steamgifts if possible to collect their giveaway. The mods will usually need this as proof that you tried to contact them. If they still don't respond, send a ticket asking for a reroll.

You must wait 7 days after sending the key.
If they have not replied, you can ask support to select a new winner.

In any case your level will not increase with rpg maker as it is considered as 0 cv as it was given for free.

i guess they too are waiting for you to send the key not knowing that you sent them via mail.
many ppl use more than one email. some specifically to register at places they dont want other ppl to know their main email... so they might have not looked inside there yet... or it ended up in spam.
also you havent mentioned in your giveaway description that you are going to send the key via mail .
why not just use the built-in key send function? its safe and regarded as the standard method of sending keys. i might be wrong here but maybe its still possible to add your keys to the built-in system.

what you can do... if they have made giveaways themselves at some point, go and make a comment about the fact that you send them the key via mail. they will get a notification tht they have a new message and hopefully look inside.
if they dont have own GAs then i suggest you update your GA description asap. maybe they look inside at one point and see that.
thats all if the winner are online at some point and arent bots.
also if you try to contact them via steam, dont use word like gift, won, win, giveaway and the likes. it might trigger steams algorithm and ban you.

rerolling is a bit difficult in this case i would say... if you reroll after a week and it happens that they see the mail with the key and activated them before your new winners had done so, then youll get a non-received.

hope that helps