Across the board, the biggest stumbling block is what’s in your factory right now. In most factories, there is going a combination of new and old machines. Some might have been built there in the 1980s, but they are still going strong. Updating them for IoT or replacing them, however, will be expensive. Some firms will have new machines they just purchased or relatively new machines that they retrofitted recently. So there’s going to be a huge variation in what companies have in their factories.
Another stumbling block is that some people define IoT as it being built around a network or cloud of external servers that will not be owned by individual manufacturers, but managed by someone else. If that is the case, it will be a long wait until IoT is fully in gear, because the manufacturers I talked to are not comfortable with storing and using data from the cloud or external servers. If you let IoT be based around a network owned and inside a company, you can still carry out many of the same tasks possible with external servers and everything hooked into the internet.