Kevin
I'm assuming that all three receivers exhibit exactly the same problem. If so then the logical answer, as Kip infers, is that the transmitter is faulty because it's the only common factor. One faulty Rx would be bad luck, while two would be suspicious and three is highly improbable. Have you changed or recharged the batteries in the transmitter recently?
Dave M

HS 645 is rated at 350mA "running current" @4.8v and 450mA @ 6v. Whether or not these figures represent the current under load – as opposed to just running unloaded – isn't stated on the datasheet I found. Neither value appears unduly high but the Tamco receivers may be delicate little things which are only intended for low-power servos.
The problem seems to be that when both servos are connected into a Rx then only Channel 1 works. Eliminating one or the other servo as the possible cause would seem to be a reasonable thing to do. Have you tried connecting just the HS311 into Channel 2 i.e. leave the other servo out of circuit altogether? Try also connecting just the HS645 into Channel 2 and see if that works. If channel 2 or higher still refuses to operate either of them in isolation then I'm at a loss to know what else to suggest. It is perfectly possible to run a high-power servo using its own separate [high-capacity] battery pack, but that would be a step beyond isolating the 645 as the culprit.
Sadly it's all a bit empirical but there's so little published data on these cheap Chinese radios [aka none] that there doesn't seem to be a more technical way of trouble-shooting them. I try to eliminate as many potential compatibility problems as I can by sticking to just one reputable manufacturer; in my case it's Hitec.

Dave M

A servo, if working properly, can only do as it is told by the receiver. This, in turn, only passes the message along that comes from the transmitter.

A servo getting no signal will twitch a bit on power-up, and sit there, twitching the same way at the next power-up, until it eventually gets to the end of its travel.

If it travels to the end in one go, it is getting a signal telling it to do that, and it begins to look like either the transmitter is sending an "out of range" instruction, or the receiver is translating its signal to something out of range. If waggling the stick does nothing, it could mean that the signal never gets to the point where the servo can respond.

Or there is something in the servo causing the problem. In my case, when something like that happens it is salty water where it shouldn't be altering the servo circuit.