Photo stream is used to sync photos from one device to another, not to use as a storage medium. The stream only keeps photos for up to 30 days and then deletes them. when you turned on Photo Stream on the new mac, you only got those photos that are still in the stream.
Once a photo is sent via the stream to another device, you should move the photo to a permanent album. Although as long as you keep the stream turned on on one device, its photo stream photos remain, but if that device is ever reset or the stream is turned off, then turning it on again will only retrieve the existing photos in the stream.
If you still have the originals on the iphone/ipad, you can always sync those photos to the mac..
Read this... import photos to your computer...
http://support.apple.com/kb/HT4083