If she's a first-timer then it's very possible, as they don't look full, that she isn't standing still for them, and/or they are not latching on effectively.
Get in the pen with them. First of all, check that you can get milk out of both teats - quite possibly the plug is still in there and you will need to remove it. Then hold her steady to give them the chance to suckle properly. If you have any doubt that they're latching on properly, you may need to guide them onto the teat while holding her steady - not easy, but necessary!
If you can't manage it (and it isn't easy, if she won't stand and they won't latch on) you can milk her into a jug then bottle or tube that into them for now - but you will have to get them onto her teats so it's better to get that sorted now or they'll be hungry overnight. They need to be feeding at least every 4 hours at this point, really.
If you have someone who can help you then it's much easier - one holds the ewe steady and the other guides the lamb onto the teat.
You can only be sure the lamb is on the teat if you are looking at the teat in its mouth; other signs such as 'being under her and sucking' and 'waggling its tail' are not reliable indications.
BH's tip is to count for 60 seconds for the first feed - count while the lamb is latched on and sucking effectively (see above); stop counting when it comes off and resume when it's on again. Once they've had a good bellyful, and she's got used to the feeling of it, they usually manage themselves thereafter - but the odd one still needs some help for a wee while, so do check their bellies again later on.