It could also be the result of this known bug:
https://core.trac.wordpress.org/ticket/32315
If your data is larger than the varchar field length then $wpdb->insert will fail and produce no errors.
If you don’t mind data being truncated if it’s larger than the field length, switch to $wpdb->query($wpdb->prepare(..)).
Because of this stupid bug I haven’t used insert in years.