As with any transaction, it could have been flagged for fraud/money laundering and been put on a hold for additional checks. This could have been done by both the solicitors bank and also Monzo and doesn’t matter if it was a quick CHAPS payment it would still need to be investigated.
Some banks like HSBC also don’t seem to like initial payments to Monzo and holds them up for additional checks so if your solicitors bank is HSBC, or ones like it, it could be that.
Reading a Financial Ombudsman newsletter it also gives a scenario that the receiving bank could literally be too busy to receive a CHAPS payment thus puts it on hold it up to deal with later. You would just have to wait that out I guess.
Your solicitors accidently made (or yourself) a sort code/account number error. The big one being getting the 04-00-04 sort code wrong. Some people have reported failed payments and it turns out they put incorrectly 04-00-40 by mistake.
Monzo will generally request a TRN (Transfer Reference Number)* from the sending bank so that they can chase the payment quicker from their side. Ask your solicitor to contact their bank for the TRN for you.
*There could be a CHAPS reference number but this is where my knowledge is more hazy as the above quote was for a Faster Payment. Maybe as your solicitor to ask for both a TRN and/or a CHAPS reference number and don’t give up calling Monzo.