SMART SPF is designed to pick up all your previous SPF records of your domain that have been reflected over your DNS. The aim is to aid in managing your SPF without going to the DNS server.
Your SMART SPF might not work due to multiple reasons. Some of the possibilities are as follows:
On rare occasions, it fails to pick up older sources; in that case, you will have to look up the SPF records for those sources and upload them from your end. You can do so in the SMART SPF section.
Your DNS has a lookup limit, recommended below 10. Once exceeded a PermError will be returned, you can use the SPF flattening feature offered by KDMARC. SPF Flattening will basically concise the long list of SPF records into just one SPF TXT Record.
Later, go back to ur DNS and check if there is only one SPF txt record, that is SMART SPF.
Ensure you have included all your sources under one SPF TXT record as multiple records will cause SPF failing or during the SMART SPF encapsulation process, it will encapsulate your previous SPF records. Also, in the case of multiple records, the features tend to pick up the first available SPF TXT record and leave the rest as it is, hence leading to SPF failure.