Plants will stretch when they are receiving too much shade, and when they are too hot. Be sure the greenhouse is not located where it gets shaded by trees or buildings, especially during the cloudy winter months.
Full sun is required.
However, if the greenhouse is getting over 90 degrees, it is necessary to install some type of cooling mechanism besides exhaust fans. Consider evaporative cooling pads, a fogging system, shade compound, shade cloth, or some combination of these approaches. Recommendations on this web site do not endorse any commercial products or trade names.