Are you using workspaces/partitions? If so, some leads may be in a different workspace.
Searching the default workspace may locate the "missing" leads.
Are the landing page and form generated by Design Studio or hosted on the corporate website?
If hosted, is the form submitting via SOAP API or Munchkin JavaScript API?
The failure to send the leads to Marketo may be caused by conflicting jQuery versions.
More details would be helpful to help you.