Its been about an hour or more since I created the monster and I searched up on this issue and tried the following :
- Recreate the monster again.
- Save changes and wait for about 30mins.
- Checked if it exists in "My Collections" and yes.
- Logged out and logged in and checked after 30mins.
- Checked the filters under the encounter builder to show all homebrew and unfilter everything and searched the name.
It still doesn't appear, previously made monsters are still there.

https://www.dndbeyond.com/monsters/3399354-ripper
https://www.dndbeyond.com/monsters/3399392-squee

I'm having the same issue with newly created monsters. It seems that there is some sort of cutoff date they had to be created before to show up.

I'm having the same issue here. I tried the same AstralAyano and no success :/

I'm glad I saw your post. I thought I was stupid. Today, I also tried everything to get my monster into the Encounter Builder. Because I initially created it in German, I thought that might have been a problem, but apparently, the server is acting up or something.

https://www.dndbeyond.com/monsters/3399326-klytharos-the-mountain-troll

I'm having this very same issue. Older homebrew monsters are showing up just fine, but ones I create or pull in from the community within the last 24 hours are not showing up at all. I have cleared by cache and cookies, logged off of DND Beyond, switched browsers, etc. with no change. Makes me wonder if a minor update to their code caused this bug?

 I have been trying to build a couple of encounters since yesterday.  So this is more than a 24hr bug at this point...

Yehh at first I thought I was doing something wrong since I'm new to dndbeyond but after some searches it seems some people have this as well, most other solutions I see is just to wait, but some people have waited more than 24 hours, so I'm kinda unsure what to do.

Same issue here as well. Trying to prepare for a game tomorrow but may have to go old school. 

same its been almost 24 hours for me and i still cant add it. I have a session today that i was hoping to run this monster in so imma have to go back to the old fashion way.

I am having the same issue as everyone else. Had to run an encounter last night while switching between extra tabs for my monsters.

Here I thought I was doing something wrong.  But this is annoying.

I am having the same problem.  And, this is a feature I use constantly.  I hope it gets fixed (temporary bug) very soon rather than permanent removal of the capability.  One feature I used in the past was removed already. If they were to remove another, I might have to cancel.  But, I’m hopeful it is just a temporary glitch or bug.

I am also encountering this issue. I have the same feelings as many expressed above; the encounter builder is fairly pointless if I can't include homebrew monsters.

I submitted a support request through https://dndbeyond-support.wizards.com/hc/en-us/requests/new?ticket_form_id=6098987232148 and hope it gets resolved soon. Is there another better way to report it? Maybe multiple reports will bring quicker results?

I also logged a support ticket 

https://dndbeyond-support.wizards.com/hc/en-us/requests/3222844

Replying to this thread to confirm this is has happened to my latest two homebrew monsters, both created within the past few days. 

Still happening for me just now.

ok so I am not going mad then .... 

I doubt we'll see any progress on this whilst it's the weekend as it's not going to be considered a critical call out problem for a tool which is technically in "Beta". Guess I won't be running my campaign today.

I'm glad I'm not the only one with the problem. I thought I was going crazy. It really is a critical feature for me to include homebrew monsters in the Encounter Builder. Hope this gets fixed asap.