The Shure SLX2 hand-held microphone has similar performance characteristics to the much-loved (cabled) Shure SM58 vocal microphone. It has a 'cardoid pick-up pattern' intended for close work - being spoken / sung straight into the top of the capsule at no more than about 8 inches from the mouth of the user- to minimise feedback and extraneous noise. If you double the distance between the mouth and microphone the sound level will drop four-fold so distance really matters.
The SLX1 lapel/ lavalier is a semi 'omni-directiona'l pick up pattern microphone, which should work satisfactorily up to 12 inches from the speakers mouth, but closer is better. Always try to get your users to hang it about 6 inches below their chin, at most, and preferably with the top of the microphone capsule pointed upwards towards the mouth.
Here are a few other things for you to try:

1. There is a volume (gain) knob on the back panel of the SLX receiver unit. Try turning this up to maximum (needs a small screwdriver)
2. Make sure that both aerials on the receiver are fully extended at different angles to each other and in a clear 'line-of'sight' with the transmitter pack, with no large bits of metal in between which could block the signal. Ideally the receiver should not be more than 25metres from the transmitter. If distance is a problem locate the receiver near to the transmitter and run a balanced (xlr) cable to your mixer/ amplifier.
3. The angle that the receiver aerials are at should be broadly similar to the angle that the transmitter antenna is at to optimise signal reception.
4. Make sure the receiver is not located too close to other electrical equipment which may be interfering with the signal (especially voltage transformers, CD or DVD players and hearing loop induction amplifiers which have a strong magnetic field)
5. Always use good quality batteries - Duracell Ultra or equivalent ( do not use rechargeable batteries as they seldom achieve optimum voltage)
6. Check that the antenna is tightly screwed into the SLX1 transmitter pack and not damaged.
7. The SLX2 hand-held microphone has an adjustable level switch inside the body - unscrew the top of the microphone to access it - switch to maximum.
8. Another possible cause could be an impedance mis-match between the SLX receiver and your amplifier or mixer. There are two outputs on the rear panel of the receiver. One is an XLR (large 3 pin socket) this is LOW impedance and needs connecting to a low impedance input on your mixer/amplifier. You should normally be using this output even if your amplifier has a jack-socket input (use a proper balanced XLR to 6mm trs jack cable rather than a adapter plug). The other output on the receiver unit is a 6mm HIGH impedance, unbalanced, standard jack socket (labelled 'line in'). This would normally be to connect to a guitar amplifier which has a high impedance 'line level' input socket. If you are using this output you may need to use a pre-amplifier to get a big enough signal for a microphone.