For us 'everyday athletes' our training progress will almost certainly not be linear, our progress and how it feels will vary week to week.
This shouldn't be seen as a negative! While over time a commitment to progressive strength and conditioning training coupled with the correct food and exercise will see you improve this won't necessarily be a straight path.
For most of stress coming from any number of things can affect our performance in the gym. Whether it's a busy week at work, an argument with your partner or the kids impacting your sleep these things will all affect your training at some point.
Now, this is not an 'excuse' not to train hard but it's important we keep them in mind. If you feel 'weaker' in the gym on any given day have a look back at your last couple of days and see if there's anything that may affect it, you'll be surprised just how often there's been something that will explain it.
On the flip side to this when we feel good and rested make the most of it! There will invariably be more bad days than good at certain times so when you've got energy and your head is in the right place pull the plug!