That's a fair question. First, I should note that I'm not able to provide any advice, and I won't be able to assess your specific situation to determine whether your lien is valid or invalid. However, I am able to provide some information that should be helpful. If you're still unsure about the status of your claim, it might be worthwhile to consult a local construction attorney - they'll be able to review the circumstances and advise you on the lien claim and how to move forward.
With that in mind, let's talk about liens and notices. As a general note, anytime notice is required but not sent, that failure to provide notice may very well result in an invalid or unenforceable lien filing. Of course, that doesn't necessarily mean that the lien claimant opens themselves up to liability - there's a difference between a fraudulent lien and an honest mistake. Of course, that's operating under the assumption that a preliminary notice was required in the first place (therefore, potentially creating a lien validity issue).
While it's always a good idea to send preliminary notice, in Missouri, most parties do not need to provide preliminary notice in order to preserve the right to lien. When hired directly by the property owner, contractors must provide a disclosure notice prior to receiving payments. For commercial jobs, equipment lessors must provide notice when the rental amount exceeds $5,000. There are some other, more intricate rules, and there are also rules regarding sending Notice of Intent to Lien (all parties not hired directly by owner must send), and you can read about those here: Missouri Lien & Notice FAQs.
Finally, in a situation where preliminary notice was required but not sent, and a mechanics lien was filed anyway, there are a number of things to consider. First, the mechanics lien might still give rise to payment. Mechanics liens are a drastic remedy, and when filed, liens will typically result in payment without further legal action needed. Thus, a potential issue with the lien claim could be immaterial if the other party to the dispute does not notice or if the dispute does not make it to the court room. Further, even if payment isn't immediately made, sending a document like a Notice of Intent to Foreclose (which is a threat to file a lawsuit based on the lien) can serve as another tool to help recover payment. Nobody likes dealing with mechanics liens, and everyone really hates having to deal with a lawsuit - so threatening to escalate the lien to a lawsuit could help to compel payment. If push comes to shove and it seems like enforcing the mechanics lien is the only potential next step, it might come time to explore other options to recover payment - like sending a demand letter via an attorney based on something like breach of contract, unjust enrichment, or under prompt payment laws, to name a few potential options. In the event that a mechanics lien is flawed and is challenged as a result of that flaw - it's important to remember two things: (1) not every mistake results in liability. Sometimes, an error will invalidate a lien and leave it unenforceable - but that doesn't necessarily mean that the lien will result in damages or that the lien is fraudulent. (2) When a mechanics lien is flawed and the customer or owner is threatening legal action due to the flawed lien, if a claimant fears they will face legal liability, often, a customer and/or owner will be willing to leave the matter be as long as the lien claimant releases their filed mechanics lien.